Transport: Refactored the connection pool

This patch refactors the connection pool implementation to be cleaner and more aligned with the
architecture of other official clients.

It introduces the "Selector" component, which allows to customize the logic for selecting
connections from the pool without replacing the entire connection pool implementation.

It introduces the "ConnectionPoolFunc" configuration option, as a constructor function
to be used when initializing a custom connection pool implementation.

Many connection pool methods have been renamed to tighten up the semantics (eg. `Remove()` => `OnFailure()`, etc).

The support for client metrics has been refactored as well, to better support custom connection pool implementations.

Related: #95, #100
(cherry picked from commit dcd99f5)

Author: karmi

_examples/instrumentation/expvar.go

@@ -17,7 +17,6 @@ import (
 	"time"
 
 	// Import the "expvar" and "pprof" package >>>>>>>>>>
-
 	"net/http"
 	_ "net/http/pprof"
 	// <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
@@ -121,10 +120,10 @@ func main() {
 				log.Println("███", fmt.Sprintf("\x1b[1m%s\x1b[0m", "Metrics"), strings.Repeat("█", tWidth-12))
 				log.Printf(
 					""+
-						"    \x1b[2mRequests:  \x1b[0m %d\n"+
-						"    \x1b[2mFailures:  \x1b[0m %d\n"+
-						"    \x1b[2mLive nodes:\x1b[0m %s",
-					m.Requests, m.Failures, m.Live)
+						"    \x1b[2mRequests:   \x1b[0m %d\n"+
+						"    \x1b[2mFailures:   \x1b[0m %d\n"+
+						"    \x1b[2mConnections:\x1b[0m %s",
+					m.Requests, m.Failures, m.Connections)
 				log.Println(strings.Repeat("─", tWidth))
 			}
 		}

elasticsearch.go

@@ -47,8 +47,12 @@ type Config struct {
 
 	RetryBackoff func(attempt int) time.Duration // Optional backoff duration. Default: nil.
 
-	Transport http.RoundTripper  // The HTTP transport object.
-	Logger    estransport.Logger // The logger object.
+	Transport http.RoundTripper    // The HTTP transport object.
+	Logger    estransport.Logger   // The logger object.
+	Selector  estransport.Selector // The selector object.
+
+	// Optional constructor function for a custom ConnectionPool. Default: nil.
+	ConnectionPoolFunc func([]*estransport.Connection, estransport.Selector) estransport.ConnectionPool
 }
 
 // Client represents the Elasticsearch client.
@@ -135,8 +139,10 @@ func NewClient(cfg Config) (*Client, error) {
 		EnableMetrics:     cfg.EnableMetrics,
 		EnableDebugLogger: cfg.EnableDebugLogger,
 
-		Transport: cfg.Transport,
-		Logger:    cfg.Logger,
+		Transport:          cfg.Transport,
+		Logger:             cfg.Logger,
+		Selector:           cfg.Selector,
+		ConnectionPoolFunc: cfg.ConnectionPoolFunc,
 	})
 
 	return &Client{Transport: tp, API: esapi.New(tp)}, nil