[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
• [x] Covered by existing CI
• [ ] Integration tested

Checklist

• [ ] Documentation update

Reviewers

@hzding621 @qiyang0221