metrics: add a LabelMap type for variables with 1 label dimension.

This lets us publish sets of vars that are breakdowns along one
dimension in a format that Prometheus and Grafana natively know
how to do useful things with.

Signed-off-by: David Anderson <dave@natulte.net>

metrics/metrics.go

@@ -8,7 +8,7 @@ package metrics
 
 import "expvar"
 
-// Map is a string-to-Var map variable that satisfies the expvar.Var
+// Set is a string-to-Var map variable that satisfies the expvar.Var
 // interface.
 //
 // Semantically, this is mapped by tsweb's Prometheus exporter as a
@@ -21,3 +21,22 @@ import "expvar"
 type Set struct {
 	expvar.Map
 }
+
+// LabelMap is a string-to-Var map variable that satisfies the
+// expvar.Var interface.
+//
+// Semantically, this is mapped by tsweb's Prometheus exporter as a
+// collection of variables with the same name, with a varying label
+// value. Use this to export things that are intuitively breakdowns
+// into different buckets.
+type LabelMap struct {
+	Label string
+	expvar.Map
+}
+
+// Get returns a direct pointer to the expvar.Int for key, creating it
+// if necessary.
+func (m *LabelMap) Get(key string) *expvar.Int {
+	m.Add(key, 0)
+	return m.Map.Get(key).(*expvar.Int)
+}