Audit: check if context is already cancelled when assessing viability for audit

[peteski22]

Description

When the audit system is asked to log a request or response, it now checks the viability of the existing supplied context. The context is considered unviable if it has <5s remaining on its deadline, and a new context is created for use.

However, we're seeing issues where some users are getting audit log failures as the incoming context is already cancelled entirely. This means that audit will fail and report telemetry etc. which isn't what users want.

This PR includes the cancelled context in its evaluation of viability, and will create a new context with a 5s timeout if it is cancelled.

NOTE: We don't just return immediately from LogRequest and LogResponse if the context was cancelled, as we want to make every effort to capture that Vault was asked to service a request - in the audit log.

Should resolve: #27521

[github-actions]

CI Results:
All Go tests succeeded! ✅

[VioletHynes]

Nice! Looks good to me :)

[github-actions]

Build Results:
All builds succeeded! ✅