[Core] Fix the segfault from Opencensus upon shutdown (#36906)

[rkooo567]

Seems like client_ in the metrics exporter is not lock-protected, but it is accessed by 2 thread (by a metrics export thread + task execution thread when ExportNow is called). I added a lock. I am not 100% sure if this will fix the issue because I was unable to reproduce the issue, but we can ask the user to try after merging this PR.

Why are these changes needed?

Handles #36885

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(