admission: fix naming and units in exhausted metrics

[aadityasondhi]

Recently, there has been toil around the naming and the units used in the exhausted duration metrics. The elasticCPUNanosExhaustedDuration should really be called elasticCPUExhaustedDuration. The units for the following should be nanoseconds, instead of microseconds:

• elasticCPUNanosExhaustedDuration
• admission.granter.slots_exhausted_duration.kv
• admission.granter.io_tokens_exhausted_duration.kv

Internal threads:

• elastic CPU
• others

Jira issue: CRDB-38812

[aadityasondhi]

Internally, we discussed that it may be cumbersome to do this. Even though there were no strict dependencies to the metric, it may cause confusion in any investigations that look at this metric using Advanced Debug or Grafana since the raw values will go up 1000x from micros to nanos.

Decided not to do this.