ref: simplify runtime metric label setting

Use the already parsed/normalized value for the log level directly from
the flag as the value to set for `log_level` label in runtime info
metric. simpler than asking for the logger to regurgitate and stringify
it for us, especially since the stringer for slog.Level returns it in
all caps, forcing the need to re-normalize it to lower-case
:upside-face: :eye-twitch:

cmd/mango/mango.go

@@ -481,7 +481,7 @@ func main() {
 	}
 
 	// update runtime info metric
-	metricMangoRuntimeInfoLabels["log_level"] = strings.ToLower(logLevel.Level().String())
+	metricMangoRuntimeInfoLabels["log_level"] = logLevelFlagVal
 	metricMangoRuntimeInfo.With(metricMangoRuntimeInfoLabels).Set(1)
 
 	// parse log output format from flag