otlptracegrpc: Shutdown returns context error (#3990)

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

exporters/otlp/otlptrace/otlptracegrpc/client.go

@@ -130,13 +130,16 @@ var errAlreadyStopped = errors.New("the client is already stopped")
 // If the client has already stopped, an error will be returned describing
 // this.
 func (c *client) Stop(ctx context.Context) error {
+	// Make sure to return context error if the context is done when calling this method.
+	err := ctx.Err()
+
 	// Acquire the c.tscMu lock within the ctx lifetime.
 	acquired := make(chan struct{})
 	go func() {
 		c.tscMu.Lock()
 		close(acquired)
 	}()
-	var err error
+
 	select {
 	case <-ctx.Done():
 		// The Stop timeout is reached. Kill any remaining exports to force