Logs SDK: LoggerProvider is still functional after shutdown

[lmolkova]

Spec says

Shutdown SHOULD be called only once for each LogRecordProcessor instance. After the call to Shutdown, subsequent calls to OnEmit are not allowed. SDKs SHOULD ignore these calls gracefully, if possible.

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/sdk.md#shutdown-1

LoggerProvider remains fully functional after shutdown and there are tests that validate it

opentelemetry-python/opentelemetry-sdk/tests/logs/test_export.py

Lines 189 to 190 in ac81b5e

	with self.assertLogs(level=logging.WARNING):
	logger.warning("Log after shutdown")

While spec language is not strict and the issue is minor, it's nice to fix it to prevent bugs and side-effects in the user code.

Part of open-telemetry/community#1751