Add shuffle-read fetch metrics and surface per-operator metrics in plan display

[andygrove]

Which issue does this PR close?

Closes #1958.

Rationale for this change

Diagnosing shuffle-heavy queries (for example the SF100 TPC-H Ballista-vs-Comet gap) is currently blind. ShuffleReaderExec exposes only BaselineMetrics (elapsed_compute/output_rows), so the shuffle-read/transport cost — the time spent fetching remote partitions, the bytes moved, and the time blocked on the reduce-side in-flight governor (#1951) — is unmeasurable. Separately, the executor's LoggingMetricsCollector renders only the shuffle-writer wrapper metrics, so inner-operator compute (SortExec/SortMergeJoinExec/DataSourceExec elapsed_compute/spill) never appears even though DataFusion already collects it. Together these make it impossible to attribute where a shuffle-heavy query spends its time.

What changes are included in this PR?

1. Fetch metrics on ShuffleReaderExec. A ShuffleReadMetrics struct (mirroring the existing ShuffleWriteMetrics) recorded per output partition and threaded through send_fetch_partitions into the concurrent local/remote fetch tasks:

   • fetch_time — wall-time fetching remote partitions (Arrow-Flight fetch + buffering)
   • local_read_time — wall-time opening node-local shuffle files
   • permit_wait_time — a single combined timer over the three reduce-side governor semaphores (request + per-address + byte), i.e. backpressure wait (feat: bound shuffle fetch with a reduce-side in-flight governor #1951)
   • decoded_bytes — decoded in-memory Arrow footprint of fetched remote partitions (not compressed wire bytes)
   • fetch_requests — total remote fetch attempts issued, including retries
   • fetch_retries — of those, how many were retries
   • local_partitions / remote_partitions — the local/remote read split

   All metrics go through the standard MetricBuilder/MetricsSet machinery, so they also appear in EXPLAIN ANALYZE. The handles are Arc-backed atomics cloned into the concurrent fetch tasks (no added locking). Doc comments note that fetch_time/permit_wait_time are summed across concurrent tasks (aggregate cost, not wall-clock) and that fetch_time is additive to — not a re-slice of — the reader's existing elapsed_compute.

2. Per-operator metrics in the plan display. ShuffleWriterExec::Display now renders the child plan with metrics (DisplayableExecutionPlan::with_metrics, ShowMetrics::Aggregated) in addition to statistics, so inner-operator elapsed_compute/spill/output-rows surface in LoggingMetricsCollector output and EXPLAIN ANALYZE. (The originally-considered set_show_metrics setter does not exist on DisplayableExecutionPlan; with_metrics is the equivalent constructor and is already used elsewhere in the repo.)

Are there any user-facing changes?

No API changes. The additional metrics are visible in EXPLAIN ANALYZE output and executor metrics logs.