Added OTEL instrumentation to Fleet's internal HTTP client. (#40568)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #40564

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.

## Testing

- [x] QA'd all new/changed functionality manually


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Added OpenTelemetry instrumentation to the internal HTTP client to
provide enhanced observability through distributed tracing and metrics
collection for HTTP operations.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/40564-otel-http-client

@@ -0,0 +1 @@
+* Added OTEL instrumentation to Fleet's internal HTTP client.

pkg/fleethttp/fleethttp.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"time"
 
+	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
 	"golang.org/x/oauth2"
 )
 
@@ -71,9 +72,11 @@ func NewClient(opts ...ClientOpt) *http.Client {
 	if co.noFollow {
 		cli.CheckRedirect = noFollowRedirect
 	}
+	var baseTransport http.RoundTripper
 	if co.tlsConf != nil {
-		cli.Transport = NewTransport(WithTLSConfig(co.tlsConf))
+		baseTransport = NewTransport(WithTLSConfig(co.tlsConf))
 	}
+	cli.Transport = otelhttp.NewTransport(baseTransport)
 	if co.cookieJar != nil {
 		cli.Jar = co.cookieJar
 	}
@@ -125,11 +128,13 @@ func noFollowRedirect(*http.Request, []*http.Request) error {
 // token for authentication (as OAuth2 static token).
 func NewGithubClient() *http.Client {
 	if githubToken := os.Getenv("NETWORK_TEST_GITHUB_TOKEN"); githubToken != "" {
-		return oauth2.NewClient(context.Background(), oauth2.StaticTokenSource(
+		cli := oauth2.NewClient(context.Background(), oauth2.StaticTokenSource(
 			&oauth2.Token{
 				AccessToken: githubToken,
 			},
 		))
+		cli.Transport = otelhttp.NewTransport(cli.Transport)
+		return cli
 	}
 	return NewClient()
 }

pkg/fleethttp/fleethttp_test.go

@@ -3,18 +3,21 @@ package fleethttp
 import (
 	"crypto/tls"
 	"net/http"
+	"reflect"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
 )
 
 func TestClient(t *testing.T) {
 	cases := []struct {
 		name         string
 		opts         []ClientOpt
-		nilTransport bool
+		defaultInner bool
 		nilRedirect  bool
 		timeout      time.Duration
 	}{
@@ -31,10 +34,14 @@ func TestClient(t *testing.T) {
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
 			cli := NewClient(c.opts...)
-			if c.nilTransport {
-				assert.Nil(t, cli.Transport)
+			require.IsType(t, &otelhttp.Transport{}, cli.Transport, "outer transport should be otelhttp")
+			// Inspect the inner (base) transport wrapped by otelhttp via unsafe since the rt field is unexported.
+			rtField := reflect.ValueOf(cli.Transport).Elem().FieldByName("rt")
+			inner := *(*http.RoundTripper)(unsafe.Pointer(rtField.UnsafeAddr())) //nolint:gosec
+			if c.defaultInner {
+				assert.Equal(t, http.DefaultTransport, inner, "inner transport should be http.DefaultTransport")
 			} else {
-				assert.NotNil(t, cli.Transport)
+				assert.IsType(t, &http.Transport{}, inner, "inner transport should be a custom *http.Transport") //nolint:gocritic
 			}
 			if c.nilRedirect {
 				assert.Nil(t, cli.CheckRedirect)