otlpmetrichttp: Add WithHTTPClient option (#6752)

Follows https://github.com/open-telemetry/opentelemetry-go/pull/6688

Towards (for OTLP metric exporter):
- https://github.com/open-telemetry/opentelemetry-go/issues/4536
- https://github.com/open-telemetry/opentelemetry-go/issues/5129
- https://github.com/open-telemetry/opentelemetry-go/issues/2632

internal/shared/otlp/otlpmetric/oconf/options.go.tmpl

@@ -57,13 +57,15 @@ type (
 		Timeout     time.Duration
 		URLPath     string
 
-		// gRPC configurations
-		GRPCCredentials credentials.TransportCredentials
-
 		TemporalitySelector metric.TemporalitySelector
 		AggregationSelector metric.AggregationSelector
 
-		Proxy HTTPTransportProxyFunc
+		// gRPC configurations
+		GRPCCredentials credentials.TransportCredentials
+
+		// HTTP configurations
+		Proxy      HTTPTransportProxyFunc
+		HTTPClient *http.Client
 	}
 
 	Config struct {
@@ -373,3 +375,10 @@ func WithProxy(pf HTTPTransportProxyFunc) GenericOption {
 		return cfg
 	})
 }
+
+func WithHTTPClient(c *http.Client) GenericOption {
+	return newGenericOption(func(cfg Config) Config {
+		cfg.Metrics.HTTPClient = c
+		return cfg
+	})
+}

internal/shared/otlp/otlpmetric/oconf/options_test.go.tmpl

@@ -533,6 +533,24 @@ func TestConfigs(t *testing.T) {
 				assert.Nil(t, c.Metrics.Proxy)
 			},
 		},
+
+		// HTTP Client Tests
+		{
+			name: "Test With HTTP Client",
+			opts: []GenericOption{
+				WithHTTPClient(http.DefaultClient),
+			},
+			asserts: func(t *testing.T, c *Config, grpcOption bool) {
+				assert.Equal(t, http.DefaultClient, c.Metrics.HTTPClient)
+			},
+		},
+		{
+			name: "Test Without HTTP Client",
+			opts: []GenericOption{},
+			asserts: func(t *testing.T, c *Config, grpcOption bool) {
+				assert.Nil(t, c.Metrics.HTTPClient)
+			},
+		},
 	}
 
 	for _, tt := range tests {