[serve] Enable streaming over ServeHandles

[edoakes]

Why are these changes needed?

Adds support for handle.options(stream=True) to make streaming calls using the RayServeHandle.

This required two changes:

• Adding stream=True to the .options codepaths.
• Adding a branch in handle_request_streaming to handle calls from the HTTP proxy as well as other handle calls.

I also refactored some of the code around exception handling/metrics into a common context manager that's used by both codepaths.

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 :(

[edoakes]

#37147 causes the aiowrlock reader lock to raise an exception. It's currently commented out; once the fix for that issue is in, I'll uncomment and this can be merged.

[GeneDer]

LGTM!