Logging may leak slack webhook secret

[patrickZWY]

The current:
except Exception as slack_exc:
logger.error(f"Failed to post Slack alert: {slack_exc}")

may leak slack webhook secret in the case of an unhandled application exception that triggers notify_slack_of_exception() because we currently send slack alert through our app and whether this slack notification is successful or not, urllib3's debug request log (if successful) or the full slack_exc info (if failed) may be in log.

Reproduce this if you plan to fix this.

[patrickZWY]

An easy way to fix this is to just process part of the exception info instead of all of it.