Sidecar metrics collection over plugin gRPC has no deadline — one stuck operation hangs the whole instance /metrics

Trace the instance sidecar's plugin gRPC metrics collect path and reproduce the issue with a stuck plugin operation, then inspect how the /metrics endpoint handles collector errors. Verify that a per-collect timeout lets the endpoint return the remaining metrics and records an error when the plugin call stalls, while healthy plugin metrics continue to work.

Environment

CloudNativePG 1.30.0, plugin-barman-cloud v0.14.0, PostgreSQL 17/18 clusters, VersityGW (posix) S3 endpoint.

What happens

The instance sidecar collects plugin metrics over the plugin gRPC with no deadline on the collect path. Consequence: if any plugin operation gets stuck (in my case a retention/catalog-maintenance run that never completes against a posix-backend S3 gateway — filed separately), the next scrape blocks on the collector and the entire /metrics endpoint of that instance hangs indefinitely. Prometheus marks the target down and every metric of that instance disappears, not just the plugin's own collectors.

Observed in production: the primary instance's exporter dark for 6.7 hours while the database itself was healthy — WAL-archiver and backup-staleness alerting was blind precisely on the instance where it matters.

Expected

• A per-collect deadline (a few seconds) on the plugin metrics gRPC call.
• On timeout/error: return the rest of the metrics plus an error counter (e.g. ..._collector_errors_total), instead of hanging the whole endpoint. A misbehaving plugin should degrade its own metrics, never the instance's.

Reproduction sketch

1. ObjectStore against VersityGW (posix backend) with any retentionPolicy set.
2. Wait for a retention run to start (~30 min cadence) — it never completes against this backend.
3. Scrape the instance sidecar's metrics port: the request hangs until the client gives up; up goes to 0 for the instance.

Companion issue describes the retention hang itself; this one is about the missing deadline that turns any such hang into a full metrics outage.