tsweb: move varz handler(s) into separate modules

This splits Prometheus metric handlers exposed by tsweb into two
modules:
- `varz.Handler` exposes Prometheus metrics generated by our expvar
  converter;
- `promvarz.Handler` combines our expvar-converted metrics and native
  Prometheus metrics.

By default, tsweb will use the promvarz handler, however users can keep
using only the expvar converter. Specifically, `tailscaled` now uses
`varz.Handler` explicitly, which avoids a dependency on the
(heavyweight) Prometheus client.

Updates https://github.com/tailscale/corp/issues/10205

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tsweb/tsweb_test.go

@@ -7,21 +7,17 @@ import (
 	"bufio"
 	"context"
 	"errors"
-	"expvar"
 	"fmt"
 	"net"
 	"net/http"
 	"net/http/httptest"
-	"reflect"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/google/go-cmp/cmp"
-	"tailscale.com/metrics"
 	"tailscale.com/tstest"
 	"tailscale.com/util/vizerror"
-	"tailscale.com/version"
 )
 
 type noopHijacker struct {
@@ -370,323 +366,6 @@ func TestHTTPError_Unwrap(t *testing.T) {
 	}
 }
 
-func TestVarzHandler(t *testing.T) {
-	t.Run("globals_log", func(t *testing.T) {
-		rec := httptest.NewRecorder()
-		VarzHandler(rec, httptest.NewRequest("GET", "/", nil))
-		t.Logf("Got: %s", rec.Body.Bytes())
-	})
-
-	half := new(expvar.Float)
-	half.Set(0.5)
-
-	tests := []struct {
-		name string
-		k    string // key name
-		v    expvar.Var
-		want string
-	}{
-		{
-			"int",
-			"foo",
-			new(expvar.Int),
-			"# TYPE foo counter\nfoo 0\n",
-		},
-		{
-			"dash_in_metric_name",
-			"counter_foo-bar",
-			new(expvar.Int),
-			"# TYPE foo_bar counter\nfoo_bar 0\n",
-		},
-		{
-			"int_with_type_counter",
-			"counter_foo",
-			new(expvar.Int),
-			"# TYPE foo counter\nfoo 0\n",
-		},
-		{
-			"int_with_type_gauge",
-			"gauge_foo",
-			new(expvar.Int),
-			"# TYPE foo gauge\nfoo 0\n",
-		},
-		{
-			// For a float = 0.0, Prometheus client_golang outputs "0"
-			"float_zero",
-			"foo",
-			new(expvar.Float),
-			"# TYPE foo gauge\nfoo 0\n",
-		},
-		{
-			"float_point_5",
-			"foo",
-			half,
-			"# TYPE foo gauge\nfoo 0.5\n",
-		},
-		{
-			"float_with_type_counter",
-			"counter_foo",
-			half,
-			"# TYPE foo counter\nfoo 0.5\n",
-		},
-		{
-			"float_with_type_gauge",
-			"gauge_foo",
-			half,
-			"# TYPE foo gauge\nfoo 0.5\n",
-		},
-		{
-			"metrics_set",
-			"s",
-			&metrics.Set{
-				Map: *(func() *expvar.Map {
-					m := new(expvar.Map)
-					m.Init()
-					m.Add("foo", 1)
-					m.Add("bar", 2)
-					return m
-				})(),
-			},
-			"# TYPE s_bar counter\ns_bar 2\n# TYPE s_foo counter\ns_foo 1\n",
-		},
-		{
-			"metrics_set_TODO_gauge_type",
-			"gauge_s", // TODO(bradfitz): arguably a bug; should pass down type
-			&metrics.Set{
-				Map: *(func() *expvar.Map {
-					m := new(expvar.Map)
-					m.Init()
-					m.Add("foo", 1)
-					m.Add("bar", 2)
-					return m
-				})(),
-			},
-			"# TYPE s_bar counter\ns_bar 2\n# TYPE s_foo counter\ns_foo 1\n",
-		},
-		{
-			"expvar_map_untyped",
-			"api_status_code",
-			func() *expvar.Map {
-				m := new(expvar.Map)
-				m.Init()
-				m.Add("2xx", 100)
-				m.Add("5xx", 2)
-				return m
-			}(),
-			"api_status_code_2xx 100\napi_status_code_5xx 2\n",
-		},
-		{
-			"func_float64",
-			"counter_x",
-			expvar.Func(func() any { return float64(1.2) }),
-			"# TYPE x counter\nx 1.2\n",
-		},
-		{
-			"func_float64_gauge",
-			"gauge_y",
-			expvar.Func(func() any { return float64(1.2) }),
-			"# TYPE y gauge\ny 1.2\n",
-		},
-		{
-			"func_float64_untyped",
-			"z",
-			expvar.Func(func() any { return float64(1.2) }),
-			"z 1.2\n",
-		},
-		{
-			"metrics_label_map",
-			"counter_m",
-			&metrics.LabelMap{
-				Label: "label",
-				Map: *(func() *expvar.Map {
-					m := new(expvar.Map)
-					m.Init()
-					m.Add("foo", 1)
-					m.Add("bar", 2)
-					return m
-				})(),
-			},
-			"# TYPE m counter\nm{label=\"bar\"} 2\nm{label=\"foo\"} 1\n",
-		},
-		{
-			"metrics_label_map_untyped",
-			"control_save_config",
-			(func() *metrics.LabelMap {
-				m := &metrics.LabelMap{Label: "reason"}
-				m.Add("new", 1)
-				m.Add("updated", 1)
-				m.Add("fun", 1)
-				return m
-			})(),
-			"control_save_config{reason=\"fun\"} 1\ncontrol_save_config{reason=\"new\"} 1\ncontrol_save_config{reason=\"updated\"} 1\n",
-		},
-		{
-			"expvar_label_map",
-			"counter_labelmap_keyname_m",
-			func() *expvar.Map {
-				m := new(expvar.Map)
-				m.Init()
-				m.Add("foo", 1)
-				m.Add("bar", 2)
-				return m
-			}(),
-			"# TYPE m counter\nm{keyname=\"bar\"} 2\nm{keyname=\"foo\"} 1\n",
-		},
-		{
-			"struct_reflect",
-			"foo",
-			someExpVarWithJSONAndPromTypes(),
-			strings.TrimSpace(`
-# TYPE foo_AUint16 counter
-foo_AUint16 65535
-# TYPE foo_AnInt8 counter
-foo_AnInt8 127
-# TYPE foo_curTemp gauge
-foo_curTemp 20.6
-# TYPE foo_curX gauge
-foo_curX 3
-# TYPE foo_nestptr_bar counter
-foo_nestptr_bar 20
-# TYPE foo_nestptr_foo gauge
-foo_nestptr_foo 10
-# TYPE foo_nestvalue_bar counter
-foo_nestvalue_bar 2
-# TYPE foo_nestvalue_foo gauge
-foo_nestvalue_foo 1
-# TYPE foo_totalY counter
-foo_totalY 4
-`) + "\n",
-		},
-		{
-			"struct_reflect_nil_root",
-			"foo",
-			expvarAdapter{(*SomeStats)(nil)},
-			"",
-		},
-		{
-			"func_returning_int",
-			"num_goroutines",
-			expvar.Func(func() any { return 123 }),
-			"num_goroutines 123\n",
-		},
-		{
-			"string_version_var",
-			"foo_version",
-			expvar.Func(func() any { return "1.2.3-foo15" }),
-			"foo_version{version=\"1.2.3-foo15\"} 1\n",
-		},
-		{
-			"field_ordering",
-			"foo",
-			someExpVarWithFieldNamesSorting(),
-			strings.TrimSpace(`
-# TYPE foo_bar_a gauge
-foo_bar_a 1
-# TYPE foo_bar_b counter
-foo_bar_b 1
-# TYPE foo_foo_a gauge
-foo_foo_a 1
-# TYPE foo_foo_b counter
-foo_foo_b 1
-`) + "\n",
-		},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			tstest.Replace(t, &expvarDo, func(f func(expvar.KeyValue)) {
-				f(expvar.KeyValue{Key: tt.k, Value: tt.v})
-			})
-			rec := httptest.NewRecorder()
-			VarzHandler(rec, httptest.NewRequest("GET", "/", nil))
-			if got := rec.Body.Bytes(); string(got) != tt.want {
-				t.Errorf("mismatch\n got: %q\n%s\nwant: %q\n%s\n", got, got, tt.want, tt.want)
-			}
-		})
-	}
-}
-
-type SomeNested struct {
-	FooG int64 `json:"foo" metrictype:"gauge"`
-	BarC int64 `json:"bar" metrictype:"counter"`
-	Omit int   `json:"-" metrictype:"counter"`
-}
-
-type SomeStats struct {
-	Nested       SomeNested  `json:"nestvalue"`
-	NestedPtr    *SomeNested `json:"nestptr"`
-	NestedNilPtr *SomeNested `json:"nestnilptr"`
-	CurX         int         `json:"curX" metrictype:"gauge"`
-	NoMetricType int         `json:"noMetric" metrictype:""`
-	TotalY       int64       `json:"totalY,omitempty" metrictype:"counter"`
-	CurTemp      float64     `json:"curTemp" metrictype:"gauge"`
-	AnInt8       int8        `metrictype:"counter"`
-	AUint16      uint16      `metrictype:"counter"`
-}
-
-// someExpVarWithJSONAndPromTypes returns an expvar.Var that
-// implements PrometheusMetricsReflectRooter for TestVarzHandler.
-func someExpVarWithJSONAndPromTypes() expvar.Var {
-	st := &SomeStats{
-		Nested: SomeNested{
-			FooG: 1,
-			BarC: 2,
-			Omit: 3,
-		},
-		NestedPtr: &SomeNested{
-			FooG: 10,
-			BarC: 20,
-		},
-		CurX:    3,
-		TotalY:  4,
-		CurTemp: 20.6,
-		AnInt8:  127,
-		AUint16: 65535,
-	}
-	return expvarAdapter{st}
-}
-
-type expvarAdapter struct {
-	st *SomeStats
-}
-
-func (expvarAdapter) String() string { return "{}" } // expvar JSON; unused in test
-
-func (a expvarAdapter) PrometheusMetricsReflectRoot() any {
-	return a.st
-}
-
-// SomeTestOfFieldNamesSorting demonstrates field
-// names that are not in sorted in declaration order, to verify
-// that we sort based on field name
-type SomeTestOfFieldNamesSorting struct {
-	FooAG int64 `json:"foo_a" metrictype:"gauge"`
-	BarAG int64 `json:"bar_a" metrictype:"gauge"`
-	FooBC int64 `json:"foo_b" metrictype:"counter"`
-	BarBC int64 `json:"bar_b" metrictype:"counter"`
-}
-
-// someExpVarWithFieldNamesSorting returns an expvar.Var that
-// implements PrometheusMetricsReflectRooter for TestVarzHandler.
-func someExpVarWithFieldNamesSorting() expvar.Var {
-	st := &SomeTestOfFieldNamesSorting{
-		FooAG: 1,
-		BarAG: 1,
-		FooBC: 1,
-		BarBC: 1,
-	}
-	return expvarAdapter2{st}
-}
-
-type expvarAdapter2 struct {
-	st *SomeTestOfFieldNamesSorting
-}
-
-func (expvarAdapter2) String() string { return "{}" } // expvar JSON; unused in test
-
-func (a expvarAdapter2) PrometheusMetricsReflectRoot() any {
-	return a.st
-}
-
 func TestAcceptsEncoding(t *testing.T) {
 	tests := []struct {
 		in, enc string
@@ -756,45 +435,3 @@ func TestPort80Handler(t *testing.T) {
 		})
 	}
 }
-
-func TestSortedStructAllocs(t *testing.T) {
-	f := reflect.ValueOf(struct {
-		Foo int
-		Bar int
-		Baz int
-	}{})
-	n := testing.AllocsPerRun(1000, func() {
-		foreachExportedStructField(f, func(fieldOrJSONName, metricType string, rv reflect.Value) {
-			// Nothing.
-		})
-	})
-	if n != 0 {
-		t.Errorf("allocs = %v; want 0", n)
-	}
-}
-
-func TestVarzHandlerSorting(t *testing.T) {
-	tstest.Replace(t, &expvarDo, func(f func(expvar.KeyValue)) {
-		f(expvar.KeyValue{Key: "counter_zz", Value: new(expvar.Int)})
-		f(expvar.KeyValue{Key: "gauge_aa", Value: new(expvar.Int)})
-	})
-	rec := httptest.NewRecorder()
-	req := httptest.NewRequest("GET", "/", nil)
-	VarzHandler(rec, req)
-	got := rec.Body.Bytes()
-	const want = "# TYPE aa gauge\naa 0\n# TYPE zz counter\nzz 0\n"
-	if string(got) != want {
-		t.Errorf("got %q; want %q", got, want)
-	}
-	rec = new(httptest.ResponseRecorder) // without a body
-
-	// Lock in the current number of allocs, to prevent it from growing.
-	if !version.IsRace() {
-		allocs := int(testing.AllocsPerRun(1000, func() {
-			VarzHandler(rec, req)
-		}))
-		if max := 13; allocs > max {
-			t.Errorf("allocs = %v; want max %v", allocs, max)
-		}
-	}
-}