Add Prometheus Exporter for Exemplars

[czhang771]

Description

Fixes #2407
Follow-up of #4094

• Add exemplars to histogram in Prometheus exporter
• Fix typing of span_id and trace_id in exemplar related code (they were set as str when they actually are int)

Note

The Prometheus exporter seems to be implemented differently in Python compared to Java and Dotnet and uses Metric Families and a custom collector rather than directing converting the OTEL types to Prometheus types.
My current attempt focuses on the Histogram and adds the exemplars to each bucket for the histogram although I am not sure if this is the best way to go about it.

Relevant Resources:

• Prometheus Metric Protofiles: https://github.com/prometheus/client_model/blob/master/io/prometheus/client/metrics.proto
• Java Prometheus Exporter: https://github.com/open-telemetry/opentelemetry-java/tree/main/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus
• Metric Family Class Types: https://github.com/prometheus/client_python/blob/bed7432fad70a16acff5e7b03a65eb9ec350d3e6/prometheus_client/metrics_core.py#L178
• Custom Collectors in Prometheus: https://prometheus.github.io/client_python/collector/custom/

Type of change

New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Add a test to check histogram exemplar are exported with the openmetrics format: exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py::TestPrometheusMetricReader::test_histogram_with_exemplar_to_prometheus

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[fcollonval]

@czhang771 would you mind turning this pull request as ready and update the description with:

# Description

Fixes #2407
Follow-up of https://github.com/open-telemetry/opentelemetry-python/pull/4094

- Add exemplars to histogram in Prometheus exporter
- Fix typing of `span_id` and `trace_id` in exemplar related code (they were set as `str` when they actually are `int`)

> [!NOTE]
> The Prometheus exporter seems to be implemented differently in Python compared to Java and Dotnet and uses Metric Families and a custom collector rather than directing converting the OTEL types to Prometheus types.
> My current attempt focuses on the Histogram and adds the exemplars to each bucket for the histogram although I am not sure if this is the best way to go about it.

Relevant Resources:
- Prometheus Metric Protofiles: https://github.com/prometheus/client_model/blob/master/io/prometheus/client/metrics.proto
- Java Prometheus Exporter: https://github.com/open-telemetry/opentelemetry-java/tree/main/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus
- Metric Family Class Types: https://github.com/prometheus/client_python/blob/bed7432fad70a16acff5e7b03a65eb9ec350d3e6/prometheus_client/metrics_core.py#L178
- Custom Collectors in Prometheus: https://prometheus.github.io/client_python/collector/custom/

## Type of change

New feature (non-breaking change which adds functionality)

# How Has This Been Tested?

Add a test to check histogram exemplar are exported with the openmetrics format: `exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py::TestPrometheusMetricReader::test_histogram_with_exemplar_to_prometheus`

# Does This PR Require a Contrib Repo Change?

- [ ] Yes. - Link to PR:
- [x] No.

# Checklist:

- [x] Followed the style guidelines of this project
- [x] Changelogs have been updated
- [x] Unit tests have been added
- [x] Documentation has been updated

[xrmx]

The sdk part LGTM but I think we are missing a test to assert that sometimes these fields are not serialized

[xrmx]

The sdk part LGTM but I think we are missing a test to assert that sometimes these fields are not serialized

We need two tests because AFAICS we don't have any tests serializing exemplars in oltp

[fcollonval]

I rebased this PR on main following #4251. @xrmx does your comments still apply to the remaining changes?

[xrmx]

I rebased this PR on main following #4251. @xrmx does your comments still apply to the remaining changes?

Nope, they were for the sdk changes