Turn off Error Prone warning MutableMethodReturnType.
Turn off Error Prone warning MutableMethodReturnType.

MutableMethodReturnType suggests returning Guava immutable collections from API
methods, but OpenCensus doesn't expose Guava in its API. See
https://github.com/google/error-prone/issues/982.

songy23/instrumentation-java,songy23/instrumentation-java,sebright/opencensus-java,sebright/opencensus-java,census-instrumentation/opencensus-java,sebright/opencensus-java,bogdandrutu/opencensus-java,sebright/instrumentation-java,bogdandrutu/instrumentation-java,census-instrumentation/opencensus-java,census-instrumentation/opencensus-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,bogdandrutu/instrumentation-java,sebright/instrumentation-java,bogdandrutu/opencensus-java