Persist metric audit - #38664
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`mz_persist_shard_live_writers` and `mz_persist_shard_rewrite_part_count` appear on no dashboard. `mz_persist_shard_inline_backpressure_count` was registered together with inline writes but never incremented, so the panel that plots it has always shown zero. Every per-shard family costs one series per shard in every process that holds a handle to that shard, which is where the persist share of the environmentd /metrics payload comes from.
Six per-shard families were only ever read summed over all shards, so
the shard label bought nothing but one series per shard per process.
The three `mz_persist_backpressure_*` families keep their names and lose
their labels. They are set by the persist_source backpressure operator,
which never runs in environmentd, so there every one of those series was
a zero. The gauge among them now reads as the sum over live operator
instances, each contributing its most recent value through
`GaugeContribution` and withdrawing it on drop.
The operator's metric bundle, `BackpressureOperatorMetrics`, is shared
with the upsert operator. It now holds plain handles, and each owner
keeps its own series alive: the persist client through its process-level
`BackpressureMetrics`, upsert through the new per-worker
`UpsertBackpressureMetrics`.
The three `mz_persist_shard_pubsub_diff_*` counters move next to the
other pubsub receiver metrics as
`mz_persist_pubsub_client_receiver_diff_{applied,not_applied_stale,not_applied_out_of_order}`.
`mz_persist_shard_since`, `mz_persist_shard_upper`, `mz_persist_shard_stale_version`, `mz_persist_shard_schema_registry_version_count`, `mz_persist_shard_inline_part_bytes` and `mz_persist_shard_batch_part_version_bytes` appear on no curated dashboard, only on the everything dump. Frontiers per collection are in `mz_internal.mz_frontiers`, `mz_persist_shard_count` already counts shards, and each of the two `_bytes` families has a `_count` sibling on the shard dashboard. The release sign-off reference counted `mz_persist_shard_upper` series to get a shard count and now points at `mz_persist_shard_count`.
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Breakdown and explanation why we're removing what we're removing:
https://claude.ai/code/artifact/cadbde1e-53ea-4e1d-a7a1-197c799e65c9