Add support of Host header

.chloggen/otlphttpexporter_hostheader.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: enhancement
+
+# 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: Add support of Host header
+
+# One or more tracking issues or pull requests related to the change
+issues: [9395]
+
+# (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:
+
+# 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: [user]

config/confighttp/README.md

@@ -16,7 +16,10 @@ README](../configtls/README.md).
 
 - `endpoint`: address:port
 - [`tls`](../configtls/README.md)
-- `headers`: name/value pairs added to the HTTP request headers
+- [`headers`](https://pkg.go.dev/net/http#Request): name/value pairs added to the HTTP request headers
+  - certain headers such as Content-Length and Connection are automatically written when needed and values in Header may be ignored.
+  - `Host` header is automatically derived from `endpoint` value. However, this automatic assignment can be overridden by explicitly setting the Host field in the headers field.
+  - if `Host` header is provided then it overrides `Host` field in [Request](https://pkg.go.dev/net/http#Request) which results as an override of `Host` header value.
 - [`read_buffer_size`](https://golang.org/pkg/net/http/#Transport)
 - [`timeout`](https://golang.org/pkg/net/http/#Client)
 - [`write_buffer_size`](https://golang.org/pkg/net/http/#Transport)

config/confighttp/confighttp.go

@@ -236,9 +236,16 @@ type headerRoundTripper struct {
 
 // RoundTrip is a custom RoundTripper that adds headers to the request.
 func (interceptor *headerRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	// Set Host header if provided
+	hostHeader, found := interceptor.headers["Host"]
+	if found && hostHeader != "" {
+		// `Host` field should be set to override default `Host` header value which is Endpoint
+		req.Host = string(hostHeader)
+	}
 	for k, v := range interceptor.headers {
 		req.Header.Set(k, string(v))
 	}
+
 	// Send the request to next transport.
 	return interceptor.transport.RoundTrip(req)
 }

config/confighttp/confighttp_test.go

@@ -1061,6 +1061,41 @@ func TestHttpClientHeaders(t *testing.T) {
 	}
 }
 
+func TestHttpClientHostHeader(t *testing.T) {
+	hostHeader := "th"
+	tt := struct {
+		name    string
+		headers map[string]configopaque.String
+	}{
+		name: "with_host_header",
+		headers: map[string]configopaque.String{
+			"Host": configopaque.String(hostHeader),
+		},
+	}
+
+	t.Run(tt.name, func(t *testing.T) {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			assert.Equal(t, hostHeader, r.Host)
+			w.WriteHeader(http.StatusOK)
+		}))
+		defer server.Close()
+		serverURL, _ := url.Parse(server.URL)
+		setting := HTTPClientSettings{
+			Endpoint:        serverURL.String(),
+			TLSSetting:      configtls.TLSClientSetting{},
+			ReadBufferSize:  0,
+			WriteBufferSize: 0,
+			Timeout:         0,
+			Headers:         tt.headers,
+		}
+		client, _ := setting.ToClient(componenttest.NewNopHost(), componenttest.NewNopTelemetrySettings())
+		req, err := http.NewRequest(http.MethodGet, setting.Endpoint, nil)
+		assert.NoError(t, err)
+		_, err = client.Do(req)
+		assert.NoError(t, err)
+	})
+}
+
 func TestContextWithClient(t *testing.T) {
 	testCases := []struct {
 		desc       string