feat(sdk): implement exporter metrics

[anuraaga]

Description

I am helping to implement SDK internal metrics

https://opentelemetry.io/docs/specs/semconv/otel/sdk-metrics/

This implements the exporter metrics.

Similar PR in JS: open-telemetry/opentelemetry-js#6480

/cc @xrmx to help with review. As there are many exporters, this is a pretty big one

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests

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

[anuraaga]

I have applied the cleanups except for the method change since I couldn't see how to do it and make things significantly cleaner.

I will merge main after #5015

[anuraaga]

Now fails with this which is also somewhat unrelated to this PR. #5030 (review)

self = <opentelemetry.exporter.otlp.proto.http.metric_exporter.OTLPMetricExporter object at 0x1090b3860>, metrics_data = 'foo', timeout_millis = 10000, kwargs = {}
num_items = 0

    def export(
        self,
        metrics_data: MetricsData,
        timeout_millis: Optional[float] = 10000,
        **kwargs,
    ) -> MetricExportResult:
        if self._shutdown:
            _logger.warning("Exporter already shutdown, ignoring batch")
            return MetricExportResult.FAILURE

        num_items = 0
>       for resource_metrics in metrics_data.resource_metrics:
E       AttributeError: 'str' object has no attribute 'resource_metrics'

exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py:349: AttributeError

Caused by https://github.com/open-telemetry/opentelemetry-python/blob/main/exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py#L929

[anuraaga]

@xrmx Thanks - think this is finally ready (test failure looks flaky)