derp: add counters to track the type of dropped packets.

Signed-off-by: David Anderson <danderson@tailscale.com>

derp/dropreason_string.go

@@ -0,0 +1,28 @@
+// Code generated by "stringer -type=dropReason -trimprefix=dropReason"; DO NOT EDIT.
+
+package derp
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[dropReasonUnknownDest-0]
+	_ = x[dropReasonUnknownDestOnFwd-1]
+	_ = x[dropReasonGone-2]
+	_ = x[dropReasonQueueHead-3]
+	_ = x[dropReasonQueueTail-4]
+	_ = x[dropReasonWriteError-5]
+}
+
+const _dropReason_name = "UnknownDestUnknownDestOnFwdGoneQueueHeadQueueTailWriteError"
+
+var _dropReason_index = [...]uint8{0, 11, 27, 31, 40, 49, 59}
+
+func (i dropReason) String() string {
+	if i < 0 || i >= dropReason(len(_dropReason_index)-1) {
+		return "dropReason(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _dropReason_name[_dropReason_index[i]:_dropReason_index[i+1]]
+}