[chore] cleanup confighttp tests (#11176)

#### Description

Minor life improvements when reviewing the tests,

- Using stdlib for error reading
- Using the config client from test server
- Moving examples into their own file for easy discovery/maintainability

#### Link to tracking issue
N/A

config/confighttp/compression_test.go

@@ -8,12 +8,14 @@ import (
 	"compress/gzip"
 	"compress/zlib"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"testing/iotest"
 
 	"github.com/golang/snappy"
 	"github.com/klauspost/compress/zstd"
@@ -142,7 +144,7 @@ func TestHTTPCustomDecompression(t *testing.T) {
 	require.NoError(t, err, "failed to create request to test handler")
 	req.Header.Set("Content-Encoding", "custom-encoding")
 
-	client := http.Client{}
+	client := srv.Client()
 	res, err := client.Do(req)
 	require.NoError(t, err)
 
@@ -260,7 +262,7 @@ func TestHTTPContentDecompressionHandler(t *testing.T) {
 			require.NoError(t, err, "failed to create request to test handler")
 			req.Header.Set("Content-Encoding", tt.encoding)
 
-			client := http.Client{}
+			client := srv.Client()
 			res, err := client.Do(req)
 			require.NoError(t, err)
 
@@ -276,18 +278,18 @@ func TestHTTPContentDecompressionHandler(t *testing.T) {
 
 func TestHTTPContentCompressionRequestWithNilBody(t *testing.T) {
 	compressedGzipBody := compressGzip(t, []byte{})
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
 		body, err := io.ReadAll(r.Body)
 		assert.NoError(t, err, "failed to read request body: %v", err)
 		assert.EqualValues(t, compressedGzipBody.Bytes(), body)
 	}))
-	defer server.Close()
+	defer srv.Close()
 
-	req, err := http.NewRequest(http.MethodGet, server.URL, nil)
+	req, err := http.NewRequest(http.MethodGet, srv.URL, nil)
 	require.NoError(t, err, "failed to create request to test handler")
 
-	client := http.Client{}
+	client := srv.Client()
 	client.Transport, err = newCompressRoundTripper(http.DefaultTransport, configcompression.TypeGzip)
 	require.NoError(t, err)
 	res, err := client.Do(req)
@@ -298,27 +300,16 @@ func TestHTTPContentCompressionRequestWithNilBody(t *testing.T) {
 	require.NoError(t, res.Body.Close(), "failed to close request body: %v", err)
 }
 
-type copyFailBody struct {
-}
-
-func (*copyFailBody) Read([]byte) (n int, err error) {
-	return 0, fmt.Errorf("read failed")
-}
-
-func (*copyFailBody) Close() error {
-	return nil
-}
-
 func TestHTTPContentCompressionCopyError(t *testing.T) {
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	}))
-	t.Cleanup(server.Close)
+	t.Cleanup(srv.Close)
 
-	req, err := http.NewRequest(http.MethodGet, server.URL, &copyFailBody{})
+	req, err := http.NewRequest(http.MethodGet, srv.URL, iotest.ErrReader(errors.New("read failed")))
 	require.NoError(t, err)
 
-	client := http.Client{}
+	client := srv.Client()
 	client.Transport, err = newCompressRoundTripper(http.DefaultTransport, configcompression.TypeGzip)
 	require.NoError(t, err)
 	_, err = client.Do(req)
@@ -334,15 +325,15 @@ func (*closeFailBody) Close() error {
 }
 
 func TestHTTPContentCompressionRequestBodyCloseError(t *testing.T) {
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	}))
-	t.Cleanup(server.Close)
+	t.Cleanup(srv.Close)
 
-	req, err := http.NewRequest(http.MethodGet, server.URL, &closeFailBody{Buffer: bytes.NewBuffer([]byte("blank"))})
+	req, err := http.NewRequest(http.MethodGet, srv.URL, &closeFailBody{Buffer: bytes.NewBuffer([]byte("blank"))})
 	require.NoError(t, err)
 
-	client := http.Client{}
+	client := srv.Client()
 	client.Transport, err = newCompressRoundTripper(http.DefaultTransport, configcompression.TypeGzip)
 	require.NoError(t, err)
 	_, err = client.Do(req)
@@ -362,7 +353,7 @@ func TestOverrideCompressionList(t *testing.T) {
 	require.NoError(t, err, "failed to create request to test handler")
 	req.Header.Set("Content-Encoding", "snappy")
 
-	client := http.Client{}
+	client := srv.Client()
 
 	// test
 	res, err := client.Do(req)

config/confighttp/confighttp_example_test.go

@@ -0,0 +1,33 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package confighttp
+
+import (
+	"context"
+	"net/http"
+
+	"go.opentelemetry.io/collector/component/componenttest"
+)
+
+func ExampleServerConfig() {
+	settings := NewDefaultServerConfig()
+	settings.Endpoint = "localhost:443"
+
+	s, err := settings.ToServer(
+		context.Background(),
+		componenttest.NewNopHost(),
+		componenttest.NewNopTelemetrySettings(),
+		http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}))
+	if err != nil {
+		panic(err)
+	}
+
+	l, err := settings.ToListener(context.Background())
+	if err != nil {
+		panic(err)
+	}
+	if err = s.Serve(l); err != nil {
+		panic(err)
+	}
+}

config/confighttp/confighttp_test.go

@@ -1011,28 +1011,6 @@ func verifyHeadersResp(t *testing.T, url string, expected map[string]configopaqu
 	}
 }
 
-func ExampleServerConfig() {
-	settings := NewDefaultServerConfig()
-	settings.Endpoint = "localhost:443"
-
-	s, err := settings.ToServer(
-		context.Background(),
-		componenttest.NewNopHost(),
-		componenttest.NewNopTelemetrySettings(),
-		http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}))
-	if err != nil {
-		panic(err)
-	}
-
-	l, err := settings.ToListener(context.Background())
-	if err != nil {
-		panic(err)
-	}
-	if err = s.Serve(l); err != nil {
-		panic(err)
-	}
-}
-
 func TestHttpClientHeaders(t *testing.T) {
 	tests := []struct {
 		name    string