Consider removing dependency on Guava from the implementations and exporters.

[sebright2]

Removing the dependency on Guava would help avoid version incompatibilities. It would also let us avoid shading Guava, which would increase jar size. This issue is a followup to the comments in #1111, especially https://github.com/census-instrumentation/opencensus-java/pull/1111#issuecomment-378809890.

[koalalam]

Any ETA on this?

In short term, do you consider using a newer version of Guava? Version "29.0-android" is a bit old and comes with vulnerability.

https://mvnrepository.com/artifact/com.google.guava/guava https://github.com/census-instrumentation/opencensus-java/blob/master/build.gradle#L165