Update .log_level to be enum

[jack-berg]

Resolves #387.

The spec OTEL_LOG_LEVEL env var describes itself as an enum, but never states the set of enum values.

This is a mistake we shouldn't repeat in declarative config. The lack of stricter semantics for the env var was probably from the desire to avoid bikeshedding about what those enum values should actually be. But we've come a long way from those early days, and have a stable log data model and proto which include severity number definition:

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/data-model.md#field-severitynumber
• https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/logs/v1/logs.proto#L87-L113

This is the obvious choice to use for .log_level.

Failing to do this would perpetuate language inconsistency around OTEL_LOG_LEVEL to the next generation of configuration, which would be a shame.

[codeboten]

Is the choice to use this over #445?

[jack-berg]

Is the choice to use this over #445?

This is independent of #445. This is about changing .log_level to be an enum. #445 is about consistency about enum casing. If we merge #445, there will be merge conflicts with this.