TUN-8422: Add metrics for capnp method calls

Adds new suite of metrics to capture the following for capnp rpcs operations:
- Method calls
- Method call failures
- Method call latencies

Each of the operations is labeled by the handler that serves the method and
the method of operation invoked. Additionally, each of these are split
between if the operation was called by a client or served.

connection/metrics.go

@@ -43,7 +43,6 @@ type localConfigMetrics struct {
 }
 
 type tunnelMetrics struct {
-	timerRetries    prometheus.Gauge
 	serverLocations *prometheus.GaugeVec
 	// locationLock is a mutex for oldServerLocations
 	locationLock sync.Mutex
@@ -351,15 +350,6 @@ func initTunnelMetrics() *tunnelMetrics {
 	)
 	prometheus.MustRegister(maxConcurrentRequestsPerTunnel)
 
-	timerRetries := prometheus.NewGauge(
-		prometheus.GaugeOpts{
-			Namespace: MetricsNamespace,
-			Subsystem: TunnelSubsystem,
-			Name:      "timer_retries",
-			Help:      "Unacknowledged heart beats count",
-		})
-	prometheus.MustRegister(timerRetries)
-
 	serverLocations := prometheus.NewGaugeVec(
 		prometheus.GaugeOpts{
 			Namespace: MetricsNamespace,
@@ -416,7 +406,6 @@ func initTunnelMetrics() *tunnelMetrics {
 	prometheus.MustRegister(registerSuccess)
 
 	return &tunnelMetrics{
-		timerRetries:        timerRetries,
 		serverLocations:     serverLocations,
 		oldServerLocations:  make(map[string]string),
 		muxerMetrics:        newMuxerMetrics(),