Use experimental metrics api

[ksmets]

As I wanted to enable both tracing and monitoring in one project, I modified the code to use the experimental metrics api.

Keep in mind that I made the following assumptions:

• keep the start and end times of the exported metrics as is
• bounds and values of histogram are of equal length

Moreover I added locking when accessing the metric descriptors (otherwise test_cloud_monitoring_exporter_integration.py would fail randomly). I introduced metrictestutil.py with convenience functions to create metrics (based on https://github.com/open-telemetry/opentelemetry-python/blob/main/tests/opentelemetry-test-utils/src/opentelemetry/test/metrictestutil.py)

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[aabmass]

Thanks for taking a stab at this. I've opened a separate PR https://github.com/GoogleCloudPlatform/opentelemetry-operations-python/pull/204 and decided to gut some of the code and tests as so much of it is no longer applicable.