Support environment variable OTEL_LOG_LEVEL

[dyladan]

When the user provides a value to OTEL_LOG_LEVEL, the diag logger should be enabled without need for further code changes.

Original discussion: open-telemetry/opentelemetry-js-api#102

[anthony-telljohann]

My company is also very interested in being able to use OTEL_LOG_LEVEL.

[richseviora]

I'm adding my voice here as someone that would like the logging to work out of the box. Or in the alternative, tell me that that's not the case so I can enable logging instead of setting the env var and wondering why it ain't working.

[osherv]

You can assign this to me if that's ready to implement

[github-actions]

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