[confighttp] Deprecate CustomRoundTripper (#10310)

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

Deprecates the `CustomRoundTripper` field on `confighttp.ClientConfig`,
which is unused outside tests in Contrib and causes errors because it
cannot be unmarshaled or marshaled. Additionally, having a
non-configurable field on a Config struct seems non-ideal.

Soft depends on
open-telemetry/opentelemetry-collector-contrib#33371
so we're not using deprecated APIs.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes #8627

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

Adapted tests to how the new way of doing this will look. It's slightly
less ergonomic (you can't load up all the settings then just run
`ToClient`), but we have no examples of this being used by any
components, so I'm reluctant to add it to the API.

.chloggen/confighttp-customroundtripper.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: deprecation
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confighttp
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Deprecate `ClientConfig.CustomRoundTripper`
+
+# One or more tracking issues or pull requests related to the change
+issues: [8627]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: Set the `Transport` field on the `*http.Client` object returned from `(ClientConfig).ToClient` instead.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

config/confighttp/confighttp.go

@@ -58,7 +58,10 @@ type ClientConfig struct {
 	Headers map[string]configopaque.String `mapstructure:"headers"`
 
 	// Custom Round Tripper to allow for individual components to intercept HTTP requests
-	CustomRoundTripper func(next http.RoundTripper) (http.RoundTripper, error)
+	//
+	// Deprecated: [v0.103.0] Set (*http.Client).Transport on the *http.Client returned from ToClient
+	// to configure this.
+	CustomRoundTripper func(next http.RoundTripper) (http.RoundTripper, error) `mapstructure:"-"`
 
 	// Auth configuration for outgoing HTTP calls.
 	Auth *configauth.Authentication `mapstructure:"auth"`

config/confighttp/confighttp_test.go

@@ -81,7 +81,6 @@ func TestAllHTTPClientSettings(t *testing.T) {
 				MaxIdleConnsPerHost:  &maxIdleConnsPerHost,
 				MaxConnsPerHost:      &maxConnsPerHost,
 				IdleConnTimeout:      &idleConnTimeout,
-				CustomRoundTripper:   func(next http.RoundTripper) (http.RoundTripper, error) { return next, nil },
 				Compression:          "",
 				DisableKeepAlives:    true,
 				HTTP2ReadIdleTimeout: idleConnTimeout,
@@ -102,7 +101,6 @@ func TestAllHTTPClientSettings(t *testing.T) {
 				MaxIdleConnsPerHost:  &maxIdleConnsPerHost,
 				MaxConnsPerHost:      &maxConnsPerHost,
 				IdleConnTimeout:      &idleConnTimeout,
-				CustomRoundTripper:   func(next http.RoundTripper) (http.RoundTripper, error) { return next, nil },
 				Compression:          "none",
 				DisableKeepAlives:    true,
 				HTTP2ReadIdleTimeout: idleConnTimeout,
@@ -123,7 +121,6 @@ func TestAllHTTPClientSettings(t *testing.T) {
 				MaxIdleConnsPerHost:  &maxIdleConnsPerHost,
 				MaxConnsPerHost:      &maxConnsPerHost,
 				IdleConnTimeout:      &idleConnTimeout,
-				CustomRoundTripper:   func(next http.RoundTripper) (http.RoundTripper, error) { return next, nil },
 				Compression:          "gzip",
 				DisableKeepAlives:    true,
 				HTTP2ReadIdleTimeout: idleConnTimeout,
@@ -144,27 +141,13 @@ func TestAllHTTPClientSettings(t *testing.T) {
 				MaxIdleConnsPerHost:  &maxIdleConnsPerHost,
 				MaxConnsPerHost:      &maxConnsPerHost,
 				IdleConnTimeout:      &idleConnTimeout,
-				CustomRoundTripper:   func(next http.RoundTripper) (http.RoundTripper, error) { return next, nil },
 				Compression:          "gzip",
 				DisableKeepAlives:    true,
 				HTTP2ReadIdleTimeout: idleConnTimeout,
 				HTTP2PingTimeout:     http2PingTimeout,
 			},
 			shouldError: false,
 		},
-		{
-			name: "error_round_tripper_returned",
-			settings: ClientConfig{
-				Endpoint: "localhost:1234",
-				TLSSetting: configtls.ClientConfig{
-					Insecure: false,
-				},
-				ReadBufferSize:     1024,
-				WriteBufferSize:    512,
-				CustomRoundTripper: func(http.RoundTripper) (http.RoundTripper, error) { return nil, errors.New("error") },
-			},
-			shouldError: true,
-		},
 	}
 
 	for _, test := range tests {
@@ -212,9 +195,8 @@ func TestPartialHTTPClientSettings(t *testing.T) {
 				TLSSetting: configtls.ClientConfig{
 					Insecure: false,
 				},
-				ReadBufferSize:     1024,
-				WriteBufferSize:    512,
-				CustomRoundTripper: func(next http.RoundTripper) (http.RoundTripper, error) { return next, nil },
+				ReadBufferSize:  1024,
+				WriteBufferSize: 512,
 			},
 			shouldError: false,
 		},
@@ -728,15 +710,14 @@ func TestHttpReception(t *testing.T) {
 				Endpoint:   prefix + ln.Addr().String(),
 				TLSSetting: *tt.tlsClientCreds,
 			}
+
+			client, errClient := hcs.ToClient(context.Background(), componenttest.NewNopHost(), component.TelemetrySettings{})
+			require.NoError(t, errClient)
+
 			if tt.forceHTTP1 {
 				expectedProto = "HTTP/1.1"
-				hcs.CustomRoundTripper = func(rt http.RoundTripper) (http.RoundTripper, error) {
-					rt.(*http.Transport).ForceAttemptHTTP2 = false
-					return rt, nil
-				}
+				client.Transport.(*http.Transport).ForceAttemptHTTP2 = false
 			}
-			client, errClient := hcs.ToClient(context.Background(), componenttest.NewNopHost(), component.TelemetrySettings{})
-			require.NoError(t, errClient)
 
 			resp, errResp := client.Get(hcs.Endpoint)
 			if tt.hasError {
@@ -1479,23 +1460,23 @@ func BenchmarkHttpRequest(b *testing.B) {
 			Endpoint:   "https://" + ln.Addr().String(),
 			TLSSetting: *tlsClientCreds,
 		}
-		if bb.forceHTTP1 {
-			hcs.CustomRoundTripper = func(rt http.RoundTripper) (http.RoundTripper, error) {
-				rt.(*http.Transport).ForceAttemptHTTP2 = false
-				return rt, nil
-			}
-		}
+
 		b.Run(bb.name, func(b *testing.B) {
 			var c *http.Client
 			if !bb.clientPerThread {
 				c, err = hcs.ToClient(context.Background(), componenttest.NewNopHost(), component.TelemetrySettings{})
 				require.NoError(b, err)
+
 			}
 			b.RunParallel(func(pb *testing.PB) {
 				if c == nil {
 					c, err = hcs.ToClient(context.Background(), componenttest.NewNopHost(), component.TelemetrySettings{})
 					require.NoError(b, err)
 				}
+				if bb.forceHTTP1 {
+					c.Transport.(*http.Transport).ForceAttemptHTTP2 = false
+				}
+
 				for pb.Next() {
 					resp, errResp := c.Get(hcs.Endpoint)
 					require.NoError(b, errResp)