Add metrics to Datadog spans

[gphat]

Summary

Adds metrics to outgoing Datadog spans.

Motivation

SSF's spans can contain metrics and so can Datadog's! The code, however, doesn't add them to the outgoing span. Now it does!

Test plan

No tests yet — mainly because I'm not sure how to serialize a protobuf into a file to update the fixture. Will ping @aditya-stripe. :)

Verified locally however:

[stripe-ci]

Gerald Rule: Copy Observability on Veneur and Unilog pull requests

cc @stripe/observability
cc @stripe/observability-stripe

[asf-stripe]

@cory - this is failing tests and needs a rebase; should we bump this from 1.7?

[gphat]

Sure! Drop away, I’ll fix later.

…

On Thu, Oct 5, 2017 at 5:58 PM Andreas Fuchs ***@***.***> wrote: @cory <https://github.com/cory> - this is failing tests and needs a rebase; should we bump this from 1.7? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#208 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AACwZ8jFRAqM9ki9t06OMre7616MrH_2ks5spVEegaJpZM4OgMg3> .

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