exporter/metric: Add support to export Distribution with Exemplars and Exponential Histograms.

[franciscovalentecastro]

This PR adds the following functionality to the exporter/metric :

• Added support to export ExponentialHistogram.
• Added support to export TypedValue Distribution (either Histogram and ExponentialHistogram) with exemplars.
• Adds exponential_histogram example with a dashboard.json to visualize it.

Details :

• https://github.com/open-telemetry/opentelemetry-go/pull/3849 added the Exemplar data type and the Exemplars field to a HistogramDataPoint.
• https://github.com/open-telemetry/opentelemetry-go/pull/4871 added experimental Exemplar sampling support.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 4.21053% with 91 lines in your changes are missing coverage. Please review.

Project coverage is 59.79%. Comparing base (87124ac) to head (1590aff).

Files	Patch %	Lines
exporter/metric/metric.go	4.21%	89 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #777 +/- ## ========================================== - Coverage 60.73% 59.79% -0.95% ========================================== Files 56 56 Lines 5649 5743 +94 ========================================== + Hits 3431 3434 +3 - Misses 2070 2159 +89 - Partials 148 150 +2 ```

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[dashpole]

The feature has been implemented in the OTel SDK: https://github.com/open-telemetry/opentelemetry-go/blob/main/sdk/metric/internal/x/README.md#exemplars

Let me know if you are interested in contributing this.

[franciscovalentecastro]

@dashpole Update on last changes to PR :

• Added dashboard.json to visualize the exponential_histogram example. This may conflict/be confused with the other dashboard.json in the above folder. Is this the correct place and/or naming ? (I imported manually to GCM and it works correctly).
• Fixed the open comments.
• Added a TODO for the missing []attachments to the exemplar exporting (AFAIU, this will add the span_id, etc, though i'm not very knowledgable about how to do this correctly).

[dashpole]

Follow-up tasks required:

• README for the example
• Unit testing
• Exemplar attachments
• SumOfSquaredDeviations

@franciscovalentecastro can you add the README? You are welcome to work on the other tasks as well if you have time. Otherwise, I'll make sure they get done. Just let me know which you'd like to tackle