Add throttle and fault tags to X-Ray

[gphat]

Summary

Adds throttle and fault capability to the X-Ray sink.

Motivation

X-Ray has three different error-like attributes but the conversion from SSFSpan to XRaySegment has nothing to use for these fields.

This adds two new config options that, when the tags are present, sets these fields.

Test plan

Added tests.

[stripe-ci]

Gerald Rule: Copy Observability on Veneur, Unilog, Falconer pull requests

cc @stripe/observability
@stripe/observability-stripe

[gphat]

(These tests pass locally for me, fwiw. I can't trigger a re-run to test for flakes.)

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}