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.)

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