[metrics]: Allow EPP to register metrics from extention (#1787)

The motivation is
llm-d/llm-d-inference-scheduler#386 that we
would like to have more metrics from llm-d inference scheduler plugin.

The current metrics implementation is bundled in the runner of EPP, so
this PR extends the runner to allow extention to register prometheus
metrics collector.

See llm-d/llm-d-inference-scheduler#405 for a
draft PR of how metrics are going to be implemented in the plugin.

I have tested locally that both EPP and inference scheduler metrics can
be exported through the pod /metrics endpoint.

Author: JeffLuoo

cmd/epp/runner/runner.go

@@ -143,6 +143,7 @@ func NewRunner() *Runner {
 type Runner struct {
 	requestControlConfig *requestcontrol.Config
 	schedulerConfig      *scheduling.SchedulerConfig
+	customCollectors     []prometheus.Collector
 }
 
 func (r *Runner) WithRequestControlConfig(requestControlConfig *requestcontrol.Config) *Runner {
@@ -155,6 +156,11 @@ func (r *Runner) WithSchedulerConfig(schedulerConfig *scheduling.SchedulerConfig
 	return r
 }
 
+func (r *Runner) WithCustomCollectors(collectors ...prometheus.Collector) *Runner {
+	r.customCollectors = collectors
+	return r
+}
+
 func (r *Runner) Run(ctx context.Context) error {
 	opts := zap.Options{
 		Development: true,
@@ -205,6 +211,9 @@ func (r *Runner) Run(ctx context.Context) error {
 
 	// --- Setup Metrics Server ---
 	customCollectors := []prometheus.Collector{collectors.NewInferencePoolMetricsCollector(datastore)}
+	if r.customCollectors != nil {
+		customCollectors = append(customCollectors, r.customCollectors...)
+	}
 	metrics.Register(customCollectors...)
 	metrics.RecordInferenceExtensionInfo(version.CommitSHA, version.BuildRef)
 	// Register metrics handler.