admission: promote key admission control metrics to Essential

[kevin-v-ngo]

Previously, several critical admission control metrics lacked Essential status, making them harder to discover for monitoring and troubleshooting cluster health issues.

This commit promotes 9 admission control metrics to Essential:

• admission.wait_durations.* (sql-kv-response, sql-sql-response, elastic-stores, elastic-cpu) - latency histograms showing throttling delays
• admission.granter.*_exhausted_duration.kv (slots, io_tokens, elastic_io_tokens) - duration metrics indicating resource exhaustion
• admission.elastic_cpu.nanos_exhausted_duration - elastic CPU token exhaustion tracking
• kvflowcontrol.eval_wait.*.duration - flow token wait times
• kvflowcontrol.send_queue.bytes - queued raft entry size

Each metric now includes:

• Visibility: ESSENTIAL for documentation and end user monitoring inclusion
• Category: OVERLOAD
• HowToUse: guidance on interpreting values and troubleshooting

Resolves: #162508
Epic: CRDB-35697

Release note (ops change): Promoted 9 admission control metrics to Essential status, making them more discoverable in documentation and downstream dashboards. These metrics track admission control wait times, resource exhaustion (slots, I/O tokens, CPU tokens), and replication flow control, providing critical visibility into cluster health and performance throttling.

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