Stop counting client cancels as errors for the SLO

[mapno]

What this PR does:

Makes context.Cancel errors count as successes for the SLO.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

I don't believe there's a way to know that a context.Canceled error is from the client. If we cancel context in the pipeline and let that error bubble up it would look the same here. I'm not opposed to this, but I think we should throw in some tests to feel confident we don't ever bubble context canceled up here?

in the near-ish future we should move SLO calculations out a layer in cloud to avoid issues like this.