deps: adding google-auth-library-credentials

Adding this resolves the dependency conflict of missing com.google.auth.Retryable class.

Author: suztomo

trace/pom.xml

@@ -72,6 +72,11 @@
  <artifactId>gax-grpc</artifactId>
  <version>2.7.0</version>
  </dependency>
+ <dependency>
+ <groupId>com.google.auth</groupId>
+ <artifactId>google-auth-library-credentials</artifactId>
+ <version>1.8.1</version>
+ </dependency>
  <dependency>
  <groupId>com.google.auth</groupId>
  <artifactId>google-auth-library-oauth2-http</artifactId>