add new metrics for the internal client-go transport generator

Add two new metrics to monitor the client-go logic that
generate http.Transports for the clients.

- rest_client_transport_cache_entries is a gauge metrics
with the number of existin entries in the internal cache

- rest_client_transport_create_calls_total is a counter
that increments each time a new transport is created, storing
the result of the operation needed to generate it: hit, miss
or uncacheable

Change-Id: I2d8bde25281153d8f8e8faa249385edde3c1cb39

Kubernetes-commit: 3f3e1d507d081528ce8081f0daf4446d8a3b9bc0

tools/metrics/metrics.go

@@ -64,6 +64,17 @@ type RetryMetric interface {
 	IncrementRetry(ctx context.Context, code string, method string, host string)
 }
 
+// TransportCacheMetric shows the number of entries in the internal transport cache
+type TransportCacheMetric interface {
+	Observe(value int)
+}
+
+// TransportCreateCallsMetric counts the number of times a transport is created
+// partitioned by the result of the cache: hit, miss, uncacheable
+type TransportCreateCallsMetric interface {
+	Increment(result string)
+}
+
 var (
 	// ClientCertExpiry is the expiry time of a client certificate
 	ClientCertExpiry ExpiryMetric = noopExpiry{}
@@ -85,6 +96,12 @@ var (
 	// RequestRetry is the retry metric that tracks the number of
 	// retries sent to the server.
 	RequestRetry RetryMetric = noopRetry{}
+	// TransportCacheEntries is the metric that tracks the number of entries in the
+	// internal transport cache.
+	TransportCacheEntries TransportCacheMetric = noopTransportCache{}
+	// TransportCreateCalls is the metric that counts the number of times a new transport
+	// is created
+	TransportCreateCalls TransportCreateCallsMetric = noopTransportCreateCalls{}
 )
 
 // RegisterOpts contains all the metrics to register. Metrics may be nil.
@@ -98,6 +115,8 @@ type RegisterOpts struct {
 	RequestResult         ResultMetric
 	ExecPluginCalls       CallsMetric
 	RequestRetry          RetryMetric
+	TransportCacheEntries TransportCacheMetric
+	TransportCreateCalls  TransportCreateCallsMetric
 }
 
 // Register registers metrics for the rest client to use. This can
@@ -131,6 +150,12 @@ func Register(opts RegisterOpts) {
 		if opts.RequestRetry != nil {
 			RequestRetry = opts.RequestRetry
 		}
+		if opts.TransportCacheEntries != nil {
+			TransportCacheEntries = opts.TransportCacheEntries
+		}
+		if opts.TransportCreateCalls != nil {
+			TransportCreateCalls = opts.TransportCreateCalls
+		}
 	})
 }
 
@@ -161,3 +186,11 @@ func (noopCalls) Increment(int, string) {}
 type noopRetry struct{}
 
 func (noopRetry) IncrementRetry(context.Context, string, string, string) {}
+
+type noopTransportCache struct{}
+
+func (noopTransportCache) Observe(int) {}
+
+type noopTransportCreateCalls struct{}
+
+func (noopTransportCreateCalls) Increment(string) {}