Docs: explain how diag log level and OTEL_LOG_LEVEL env variable interact

[sfishel-splunk]

reading this docs section: https://github.com/signalfx/splunk-otel-js/blob/main/docs/troubleshooting.md

there is a log level being passed as the second argument to diag.setLogger, and there is mention of the OTEL_LOG_LEVEL environment variable. it's not clear if/how those two things interact

[seemk]

There's a new logLevel option now, when set it will set up the diagnostic logger with the given log level.

OTEL_LOG_LEVEL will now do the same (with the programmatic way having preference). See https://github.com/signalfx/splunk-otel-js/blob/main/docs/troubleshooting.md