Add discovery response metrics to Agent

[johnsonshih]

What this PR does / why we need it:
This PR adds two Prometheus metrics to Agent.

• akri_discovery_response_time: histogram of the discovery response latency from the time a Configuration is created/updated to the request sent to discovery handler
• akri_discovery_response_result: counter of the success/fail results when Agent invokes discover() to discovery handler

Special notes for your reviewer:

If applicable:

• this PR has an associated PR with documentation in akri-docs
• this PR contains unit tests
• added code adheres to standard Rust formatting (cargo fmt)
• code builds properly (cargo build)
• code is free of common mistakes (cargo clippy)
• all Akri tests succeed (cargo test)
• inline documentation builds (cargo doc)
• all commits pass the DCO bot check by being signed off -- see the failing DCO check for instructions on how to retroactively sign commits

[diconico07]

Looks good to me, don't forget to do the corresponding doc PR adding the new metrics there as well.

[johnsonshih]

Looks good to me, don't forget to do the corresponding doc PR adding the new metrics there as well.

sure, will push a doc update pr for the new metrics

[johnsonshih]

/version patch