fix: use debug logging level for OpenTelemetry message

[tswast]

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Fixes#441 🦕

[tswast]

Updated based on apache/airflow#13131 to not warn at import time.

[plamut]

I agree that the warning should not be issued at import time, but instead when trying to actually use BigQuery (discussion).

DEBUG level is probably indeed better, as opentelemetry is not used by default (it's listed under extras), thus shouldn't bother users that don't choose to install it.

We should choose a different name for the flag, though, IMHO.

[plamut]

(nit) It's a bit unexpected to change a "constant", perhaps rename it? Also, since it's an internal state-keeping flag that users should neither know about nor modify it, we should probably mark it as such (e.g. _warned_telemetry)

[tswast]

Good call. I've renamed in the latest commit.