chore: use default confighttp struct for tests (open-telemetry#36681)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

These components are constructing `confighttp.ClientConfig` field by
field and adding the corresponding defaults. Instead of using default
values to construct a new struct, the new struct should already contain
the defaults.

This becomes an issue when adding new fields with defaults in the
`ClientConfig` structure, see:
open-telemetry/opentelemetry-collector#10877

Error in the core collector CI:

```
=== FAIL: . TestLoadConfig/elasticsearch (re-run 1) (0.00s)
    config_test.go:200: Config mismatch (-expected +actual):
          &elasticsearchreceiver.Config{
          	ControllerConfig: {CollectionInterval: s"2m0s", InitialDelay: s"1s"},
          	ClientConfig: confighttp.ClientConfig{
          		... // 15 identical fields
          		HTTP2PingTimeout: s"0s",
          		Cookies:          nil,
        - 		MaxRedirects:     0,
        + 		MaxRedirects:     10,
          	},
          	MetricsBuilderConfig: {Metrics: {ElasticsearchBreakerMemoryEstimated: {Enabled: true}, ElasticsearchBreakerMemoryLimit: {Enabled: true}, ElasticsearchBreakerTripped: {Enabled: true}, ElasticsearchClusterDataNodes: {Enabled: true}, ...}, ResourceAttributes: {ElasticsearchClusterName: {Enabled: true}, ElasticsearchIndexName: {Enabled: true}, ElasticsearchNodeName: {Enabled: true}, ElasticsearchNodeVersion: {Enabled: true}}},
          	Nodes:                {"_local"},
          	... // 4 identical fields
          }
```
          

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes

<!--Describe what testing was performed and which tests were added.-->
#### Testing

<!--Describe the documentation added.-->
#### Documentation

<!--Please delete paragraphs that you did not use before submitting.-->

exporter/alertmanagerexporter/config_test.go

@@ -4,7 +4,6 @@
 package alertmanagerexporter
 
 import (
-	"net/http"
 	"path/filepath"
 	"testing"
 	"time"
@@ -24,7 +23,6 @@ import (
 )
 
 func TestLoadConfig(t *testing.T) {
-	defaultTransport := http.DefaultTransport.(*http.Transport)
 	t.Parallel()
 
 	cm, err := confmaptest.LoadConf(filepath.Join("testdata", "config.yaml"))
@@ -63,26 +61,24 @@ func TestLoadConfig(t *testing.T) {
 					NumConsumers: 2,
 					QueueSize:    10,
 				},
-				ClientConfig: confighttp.ClientConfig{
-					Headers: map[string]configopaque.String{
+				ClientConfig: func() confighttp.ClientConfig {
+					client := confighttp.NewDefaultClientConfig()
+					client.Headers = map[string]configopaque.String{
 						"can you have a . here?": "F0000000-0000-0000-0000-000000000000",
 						"header1":                "234",
 						"another":                "somevalue",
-					},
-					Endpoint: "a.new.alertmanager.target:9093",
-					TLSSetting: configtls.ClientConfig{
+					}
+					client.Endpoint = "a.new.alertmanager.target:9093"
+					client.TLSSetting = configtls.ClientConfig{
 						Config: configtls.Config{
 							CAFile: "/var/lib/mycert.pem",
 						},
-					},
-					ReadBufferSize:      0,
-					WriteBufferSize:     524288,
-					Timeout:             time.Second * 10,
-					MaxIdleConns:        &defaultTransport.MaxIdleConns,
-					MaxIdleConnsPerHost: &defaultTransport.MaxIdleConnsPerHost,
-					MaxConnsPerHost:     &defaultTransport.MaxConnsPerHost,
-					IdleConnTimeout:     &defaultTransport.IdleConnTimeout,
-				},
+					}
+					client.ReadBufferSize = 0
+					client.WriteBufferSize = 524288
+					client.Timeout = time.Second * 10
+					return client
+				}(),
 			},
 		},
 	}

receiver/elasticsearchreceiver/config_test.go

@@ -4,7 +4,6 @@
 package elasticsearchreceiver
 
 import (
-	"net/http"
 	"path/filepath"
 	"testing"
 	"time"
@@ -140,11 +139,6 @@ func TestValidateEndpoint(t *testing.T) {
 }
 
 func TestLoadConfig(t *testing.T) {
-	defaultMaxIdleConns := http.DefaultTransport.(*http.Transport).MaxIdleConns
-	defaultMaxIdleConnsPerHost := http.DefaultTransport.(*http.Transport).MaxIdleConnsPerHost
-	defaultMaxConnsPerHost := http.DefaultTransport.(*http.Transport).MaxConnsPerHost
-	defaultIdleConnTimeout := http.DefaultTransport.(*http.Transport).IdleConnTimeout
-
 	t.Parallel()
 
 	cm, err := confmaptest.LoadConf(filepath.Join("testdata", "config.yaml"))
@@ -173,15 +167,13 @@ func TestLoadConfig(t *testing.T) {
 				MetricsBuilderConfig: defaultMetrics,
 				Username:             "otel",
 				Password:             "password",
-				ClientConfig: confighttp.ClientConfig{
-					Timeout:             10000000000,
-					Endpoint:            "http://example.com:9200",
-					Headers:             map[string]configopaque.String{},
-					MaxIdleConns:        &defaultMaxIdleConns,
-					MaxIdleConnsPerHost: &defaultMaxIdleConnsPerHost,
-					MaxConnsPerHost:     &defaultMaxConnsPerHost,
-					IdleConnTimeout:     &defaultIdleConnTimeout,
-				},
+				ClientConfig: func() confighttp.ClientConfig {
+					client := confighttp.NewDefaultClientConfig()
+					client.Timeout = 10000000000
+					client.Endpoint = "http://example.com:9200"
+					client.Headers = map[string]configopaque.String{}
+					return client
+				}(),
 			},
 		},
 	}