Improve fuzzer outcome metrics

[vitorguidi]

Motivation

As part of the initiative to improve clusterfuzz monitoring, it is necessary to enrich some metrics regarding fuzzing outcomes.

This PR adds the platform field to the following metrics:

• FUZZER_KNOWN_CRASH_COUNT
• FUZZER_NEW_CRASH_COUNT
• FUZZER_RETURN_CODE_COUNT
• FUZZER_TOTAL_FUZZ_TIME
• JOB_KNOWN_CRASH_COUNT
• JOB_TOTAL_FUZZ_TIME
• JOB_NEW_CRASH_COUNT

It also adds the job_type field to the FUZZER_RETURN_CODE_COUNT metric.

Part of #4271