[fgerlits]

ThreadSanitizer has found a mutex lock order issue between FlowController::mutex_ (M1) and C2MetricsPublisher::metrics_mutex_ (M2): during a flow update, we lock M1 first then M2, and during heartbeats, we lock M2 first and then some metrics nodes call executeOnAllComponents(), which locks M1. This is unlikely to happen in practice, because executeOnAllComponents checks if an update is in progress before trying to acquire the M1 lock -- but the flow update could start in the tiny window between these two lines. Also, if executeOnAllComponents finds that an update is in progress, it returns early without doing its stuff, so some metrics could have been missing from the heartbeat.

Claude also found a problem with metrics node types FlowInformation, QueueMetrics and ProcessorMetrics: these store raw pointers to processors or connections inside the current flow, which can become dangling after a flow update.

This PR fixes both issues by removing executeOnAllComponents() and replacing it with a component state cache, which has its own leaf mutex. The cache is cleared before the flow update and resnapped afterwards, which happens under this mutex.

The new MetricsDuringReloadTests unit test exercises these code paths by running several metrics serializations and flow updates in parallel. Before this change, running under ThreadSanitizer, it fails with both kinds of errors (lock order inversion, invalid read); after the change, it passes.

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