adapter: add the first curated metric sinks (sizes and errors) (SQL-556)

[mtabebe]

SQL-555 landed the machinery to install curated metric sinks on every
replica, but CURATED was empty, so nothing ran at boot.

Solution:

Add the two v1 definitions: mz_metric_arrangement_sizes (three gauge
families) and mz_metric_dataflow_errors (one gauge). Both read the raw
..._raw logs, not the derived views, which re-aggregate expensively.

The sizes sink labels each series with the exported object's GlobalId, not the
timely dataflow id, which churns across re-renders. A dataflow with no export,
like a subscribe, falls back to an unattributable bucket so its bytes still count.

The errors sink reads the raw log rather than mz_compute_error_counts
because that view joins in mz_compute_dependencies, a storage-managed
source that ensure_reads_only_logs rejects: reading it would couple the
sink's frontier to envd. Elapsed stays out of v1, its log churns
continuously and cost 14% of a 2-worker replica even raw-sourced.

curated_definitions_plan only called plan_source, so a definition that
broke the log-only contract still passed the test but crash-looped the
coordinator at boot. It now runs ensure_reads_only_logs too.

Testing:

• Unit tests plan each definition and run the log-only gate.
• metric-sink.td gains a curated section and a multi-cluster,
  multi-replica section.
• metric-sink.td covers both the attributable and unattributable id paths.
• The MetricSink platform check now asserts the curated sinks survive a boot,
  which covers restart, upgrade, and zero-downtime.