Add audit metric

[andersonvcv]

Description

Adding custom audit metric for audit errors.

Motivation and Context

This is needed to track audit errors and create alerts when they happen.

How Has This Been Tested?

Built a local copy from this PR and integrated it with Reporting locally by running the binary. Used invalid audit environment variables to make the audit request fail.

Commands:

• make build
• set envs (grab from Pics containers) and include invalid audit envs
• ./oauth2-proxy --http-address=127.0.0.1:9876 --https-address=127.0.0.1:9877 --metrics-address=127.0.0.1:9090
• change evs in .env.local.worklist-gateway file in pics repo (worklist Oauth) to OAUTH_PROXY_ENDPOINT=host.docker.internal and OAUTH_PROXY_PORT=9876
• run pics