[Minor] Refactor the client Authenticators for the new "ClientAuthent…

…icator" interfaces (#5905)

* Refactor the client Authenticators to account for the new "ClientAuthenticator" interfaces

* fix linitng issue

* fixed weird formatting

extension/bearertokenauthextension/bearertokenauth.go

@@ -16,7 +16,9 @@ package bearertokenauthextension
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"net/http"
 
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/config/configauth"
@@ -47,7 +49,8 @@ type BearerTokenAuth struct {
 	logger      *zap.Logger
 }
 
-var _ configauth.GRPCClientAuthenticator = (*BearerTokenAuth)(nil)
+var _ configauth.ClientAuthenticator = (*BearerTokenAuth)(nil)
+var errNotHTTPClientAuthenticator = errors.New("requested authenticator is not a HTTP client authenticator")
 
 func newBearerTokenAuth(cfg *Config, logger *zap.Logger) *BearerTokenAuth {
 	return &BearerTokenAuth{
@@ -72,3 +75,8 @@ func (b *BearerTokenAuth) PerRPCCredentials() (credentials.PerRPCCredentials, er
 		metadata: map[string]string{"authorization": fmt.Sprintf("Bearer %s", b.tokenString)},
 	}, nil
 }
+
+// RoundTripper is not implemented by BearerTokenAuth
+func (b *BearerTokenAuth) RoundTripper(base http.RoundTripper) (http.RoundTripper, error) {
+	return nil, errNotHTTPClientAuthenticator
+}

extension/bearertokenauthextension/bearertokenauth_test.go

@@ -52,6 +52,10 @@ func TestBearerAuthenticator(t *testing.T) {
 	assert.NoError(t, err)
 	assert.NotNil(t, credential)
 
+	c, err := bauth.RoundTripper(nil)
+	assert.ErrorIs(t, err, errNotHTTPClientAuthenticator)
+	assert.Nil(t, c)
+
 	md, err := credential.GetRequestMetadata(context.Background())
 	expectedMd := map[string]string{
 		"authorization": fmt.Sprintf("Bearer %s", cfg.BearerToken),

extension/oauth2clientauthextension/extension.go

@@ -35,11 +35,8 @@ type ClientCredentialsAuthenticator struct {
 	client            *http.Client
 }
 
-// ClientCredentialsAuthenticator implements both HTTPClientAuth and GRPCClientAuth
-var (
-	_ configauth.HTTPClientAuthenticator = (*ClientCredentialsAuthenticator)(nil)
-	_ configauth.GRPCClientAuthenticator = (*ClientCredentialsAuthenticator)(nil)
-)
+// ClientCredentialsAuthenticator implements ClientAuthenticator
+var _ configauth.ClientAuthenticator = (*ClientCredentialsAuthenticator)(nil)
 
 func newClientCredentialsExtension(cfg *Config, logger *zap.Logger) (*ClientCredentialsAuthenticator, error) {
 	if cfg.ClientID == "" {