Enable env var configuration, defaulting, validation

[jeremydvoss]

• Moved defaulting to util/configuration. This will be important for unifying logic for auto and manual instrumentation.
• Added constants for reused strings, arguments, and env vars.
• Fixed some outdated tests and documentation.
  Fixes: Implement configuration through environment variables #231 Add validation for configuration options #233 Organize constants #257

New todos:

• Add env var logic to sampler Remove sampler arg env var logic once it is added to Sampler #264
• Add env var logic and validation to BatchLogRecordProcessor Remove logging export validation and env var logic once it is added to OTel SDK #265
• Change logging level to log_level to match otel env var Change logging_level to log_level #263
• Add env var samples Add env var configuration samples #266
• Design env var usage for metric readers and views Design env var configuration for views and metric readers #267

[lzchen]

Could you add more details to the issues #263 #264 and #265?

[lzchen]

Add env var logic to sampler https://github.com/microsoft/ApplicationInsights-Python/issues/263

How is changing the logging level related to the sampler?

[jeremydvoss]

Add env var logic to sampler https://github.com/microsoft/ApplicationInsights-Python/issues/263

How is changing the logging level related to the sampler?

Not log level --OTEL_TRACES_SAMPLER_ARG. Following the structure of the other otel components, I think the sampler itself should check the env var

EDIT: Oh, I see the confusion. I mismatched the issues with their descriptions in this PR. Fixed