[terraform] metric filters for error logs in identity service

Summary:
This introduces metric filters to be used by cloudwatch alarms later in the stack.

Decided to use a for_each pattern so adding new identity alarms can be as easy as adding the pattern to identity_error_patterns.

Depends on D11852

Test Plan: Activated metric filter successfully with example error logs on AWS Console

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11853