[Fetcher] Print exceptions prior to reporting for metrics

[nikhilsimha]

Summary

We now expose exceptionSampleRate via API - default is 0.1 - one among every 10 exceptions will be logged.

Why / Goal

Currently we don't log exceptions at all, which makes debugging difficult. This will make it so that by default we print a fraction of exceptions.

Test Plan

• Added Unit Tests
• Covered by existing CI
• Integration tested

Checklist

• Documentation update

Reviewers

@hzding621 @qiyang0221

[qiyang0221]

Will this guarantee at least one exception per each fetching?

[nikhilsimha]

Will this guarantee at least one exception per each fetching?

It doesn't, but you can set the rate to 1 if you want to guarantee.

[qiyang0221]

Will this guarantee at least one exception per each fetching?

It doesn't, but you can set the rate to 1 if you want to guarantee.

This might not ideal cuz we need at least one log message from AFP client to identify the root cause in a short time.

[nikhilsimha]

Will this guarantee at least one exception per each fetching?

It doesn't, but you can set the rate to 1 if you want to guarantee.

This might not ideal cuz we need at least one log message from AFP client to identify the root cause in a short time.

You can adjust the exception rate on your end.