Set otlp-proto-grpc as the default metrics exporter for auto-instrumentation

[aabmass]

Description

In opentelemetry-distro, set otlp-grpc-proto as the default value for OTLP_METRICS_EXPORTER. This make it the default when using opentelemetry-instrument, similar to tracing.

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)
• This change requires a documentation update

How Has This Been Tested?

Install the change in local venv and ran:

$ opentelemetry-instrument ipython
Python 3.9.12 (main, Mar 24 2022, 13:02:21)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.24.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from opentelemetry import metrics

In [2]: metrics.get_meter_provider()._sdk_config.metric_readers[0]._exporter
Out[2]: <opentelemetry.exporter.otlp.proto.grpc.metric_exporter.OTLPMetricExporter at 0x7fb4387ecdf0>

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• [ ] Documentation has been updated WIP, see Update opentelemetry.io documentation with metrics opentelemetry-python#2732