Add RED metrics

- this PR adds metrics via Prometheus for instrumentation and
to move towards using HPAv2 / custom metrics in Kubernetes.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

watchdog/readconfig.go

@@ -91,6 +91,8 @@ func (ReadConfig) Read(hasEnv HasEnv) WatchdogConfig {
 		cfg.combineOutput = parseBoolValue(hasEnv.Getenv("combine_output"))
 	}
 
+	cfg.metricsPort = 8081
+
 	return cfg
 }
 
@@ -132,4 +134,7 @@ type WatchdogConfig struct {
 
 	// combineOutput combines stderr and stdout in response
 	combineOutput bool
+
+	// metricsPort is the HTTP port to serve metrics on
+	metricsPort int
 }