tsweb/varz: flesh out munging of expvar keys into valid Prometheus metrics

From a problem we hit with how badger registers expvars; it broke
trunkd's exported metrics.

Updates tailscale/corp#1297

Change-Id: I42e1552e25f734c6f521b6e993d57a82849464b2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsweb/varz/varz.go

@@ -17,6 +17,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode"
+	"unicode/utf8"
 
 	"tailscale.com/metrics"
 	"tailscale.com/version"
@@ -85,8 +87,29 @@ func prometheusMetric(prefix string, key string) (string, string, string) {
 			label, key = a, b
 		}
 	}
+
+	// Convert the metric to a valid Prometheus metric name.
+	// "Metric names may contain ASCII letters, digits, underscores, and colons.
+	// It must match the regex [a-zA-Z_:][a-zA-Z0-9_:]*"
+	mapInvalidMetricRunes := func(r rune) rune {
+		if r >= 'a' && r <= 'z' ||
+			r >= 'A' && r <= 'Z' ||
+			r >= '0' && r <= '9' ||
+			r == '_' || r == ':' {
+			return r
+		}
+		if r < utf8.RuneSelf && unicode.IsPrint(r) {
+			return '_'
+		}
+		return -1
+	}
+	metricName := strings.Map(mapInvalidMetricRunes, prefix+key)
+	if metricName == "" || unicode.IsDigit(rune(metricName[0])) {
+		metricName = "_" + metricName
+	}
+
 	d := &prometheusMetricDetails{
-		Name:  strings.ReplaceAll(prefix+key, "-", "_"),
+		Name:  metricName,
 		Type:  typ,
 		Label: label,
 	}