Fix data-race in metric without code and method but with `WithLabelFromCtx` (#1318)

This commit fixes a data race that exists when the metric used in any
`promhttp` middleware doesn't collect the `code` and `method` but uses
`WithLabelFromCtx` to collect values from context.

The problem happens because when no `code` and `method` tags are
collected, the `labels` function returns a pre-initialized map
`emptyLabels` for every request.

When one or multipe `WithLabelFromCtx` options are configured, the
returned map from the `labels` function call is used to collect the
metrics from context which creates a multi-write data race.

Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

prometheus/promhttp/instrument_server.go

@@ -389,16 +389,13 @@ func isLabelCurried(c prometheus.Collector, label string) bool {
 	return true
 }
 
-// emptyLabels is a one-time allocation for non-partitioned metrics to avoid
-// unnecessary allocations on each request.
-var emptyLabels = prometheus.Labels{}
-
 func labels(code, method bool, reqMethod string, status int, extraMethods ...string) prometheus.Labels {
-	if !(code || method) {
-		return emptyLabels
-	}
 	labels := prometheus.Labels{}
 
+	if !(code || method) {
+		return labels
+	}
+
 	if code {
 		labels["code"] = sanitizeCode(status)
 	}

prometheus/promhttp/instrument_server_test.go

@@ -250,7 +250,7 @@ func TestLabels(t *testing.T) {
 	}{
 		"empty": {
 			varLabels:  []string{},
-			wantLabels: emptyLabels,
+			wantLabels: prometheus.Labels{},
 			reqMethod:  "GET",
 			respStatus: 200,
 			ok:         true,