Add fixture with a histogram exemplar, and implement exemplar attachments in SDK

[dashpole]

Fixes https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/issues/806

This makes sure exemplar support, which was fixed in https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/pull/810 displays correctly in the UI for histograms

[dashpole]

This now implements exemplar attachments as well. I had to do that to get the fixture tests to pass.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.40000% with 37 lines in your changes are missing coverage. Please review.

Project coverage is 60.07%. Comparing base (8f90874) to head (f19eae6).

Files	Patch %	Lines
exporter/metric/metric.go	17.07%	34 Missing :warning:
.../collector/integrationtest/testcases/conversion.go	95.23%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #813 +/- ## ========================================== + Coverage 59.90% 60.07% +0.17% ========================================== Files 56 56 Lines 5749 5854 +105 ========================================== + Hits 3444 3517 +73 - Misses 2155 2186 +31 - Partials 150 151 +1 ```

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[dashpole]

We have int64 histogram exemplars from the fixture test now: