metrics: Prometheus text exposition for /metrics

/metrics was Prometheus-ish name/value text with a custom banner and
text/plain Content-Type, so strict scrapers could fail.

Emit Prometheus text 0.0.4 (# HELP, # TYPE, counter|gauge) with stable
metric names, and set Content-Type to text/plain; version=0.0.4; charset=utf-8.

steamcache/handler.go

@@ -76,9 +76,9 @@ func (sc *SteamCache) handleSpecialEndpoints(w http.ResponseWriter, r *http.Requ
 	}
 
 	if r.URL.String() == "/metrics" {
-		// Return metrics in a simple text format
+		// Prometheus text exposition format 0.0.4
 		stats := sc.GetMetrics()
-		w.Header().Set("Content-Type", "text/plain")
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
 		w.WriteHeader(http.StatusOK)
 		metrics.WriteText(w, stats)
 		return true