Implement wrapper for clientcredentials.Config

Implement a wrapper for clientcredentials.Config in preparation for
custom behavior and logic when issuing new tokens.

extension/oauth2clientauthextension/clientcredentialsconfig.go

@@ -0,0 +1,48 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package oauth2clientauthextension // import "github.com/open-telemetry/opentelemetry-collector-contrib/extension/oauth2clientauthextension"
+
+import (
+	"context"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/clientcredentials"
+)
+
+// clientCredentialsConfig is a clientcredentials.Config wrapper.
+type clientCredentialsConfig struct {
+	clientcredentials.Config
+}
+
+type clientCredentialsTokenSource struct {
+	ctx    context.Context
+	config *clientCredentialsConfig
+}
+
+// clientCredentialsTokenSource implements TokenSource
+var _ oauth2.TokenSource = (*clientCredentialsTokenSource)(nil)
+
+// createConfig creates a proper clientcredentials.Config with values retrieved
+// from files, if the user has specified a "file:" prefix
+func (c *clientCredentialsConfig) createConfig() (*clientcredentials.Config, error) {
+	return &clientcredentials.Config{
+		ClientID:       c.ClientID,
+		ClientSecret:   c.ClientSecret,
+		TokenURL:       c.TokenURL,
+		Scopes:         c.Scopes,
+		EndpointParams: c.EndpointParams,
+	}, nil
+}
+
+func (c *clientCredentialsConfig) TokenSource(ctx context.Context) oauth2.TokenSource {
+	return oauth2.ReuseTokenSource(nil, clientCredentialsTokenSource{ctx: ctx, config: c})
+}
+
+func (ts clientCredentialsTokenSource) Token() (*oauth2.Token, error) {
+	cfg, err := ts.config.createConfig()
+	if err != nil {
+		return nil, err
+	}
+	return cfg.TokenSource(ts.ctx).Token()
+}