Add @SuppressWarnings("AutoValueSubclassLeaked")

[ben-manes]

I'm adding this to a new project and received the warning,

 [AutoValueSubclassLeaked] Do not refer to the autogenerated AutoValue_ class outside the file containing the corresponding @AutoValue base class.
    PolicyStats.Metric result = new AutoValue_PolicyStats_Metric(
                                    ^
    (see https://errorprone.info/bugpattern/AutoValueSubclassLeaked)

This seems to be due to the build() method. Ideally this should be suppressed directly, rather than in the error prone configuration.

[ben-manes]

I forgot to add excludedPaths = "${buildDir}/generated-sources/.*" in the new project. Still seems like a nice change though.