Update go dependencies

vendor/github.com/census-instrumentation/opencensus-proto/CONTRIBUTING.md

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+# How to submit a bug report
+
+If you received an error message, please include it and any exceptions.
+
+We commonly need to know which language you are using (e.g. Java) and what
+platform you are on:
+ * Operating system (i.e., ```uname -a```)
+
+# How to contribute
+
+We definitely welcome patches and contributions to Census! Here are some
+guideline and information about how to do so.
+
+## Before getting started
+
+In order to protect both you and ourselves, you will need to sign the
+[Contributor License Agreement](https://cla.developers.google.com/clas).
+
+We follow the [Google Proto Style Guide](
+https://developers.google.com/protocol-buffers/docs/style).
+
+## Proposing changes
+
+Make sure that `bazel build :all` completes successfully without any new warnings.
+Then create a Pull Request with your changes. When the changes are accepted, they
+will be merged or cherry-picked by a Census core developer.