Initialize ServerMetrics in PredownloadScheduler

[pradeeee]

Problem

The PredownloadScheduler runs in a separate predownload container that starts before the main Pinot server process. Its purpose is to pre-download segments from deep storage so the server can start faster.

Previously, the initializeMetricsReporter() method in PredownloadScheduler only created PredownloadMetrics but never initialized the ServerMetrics registry. As a result:

1. PredownloadMetrics internally depends on ServerMetrics.get() for reporting server-level meters and timers.
2. Since ServerMetrics was never initialized in the predownload container lifecycle, ServerMetrics.get() always returned the NOOP instance.
3. This meant no server-level metrics (download latency, failure counts, etc.) were emitted from the predownload container — metrics were silently dropped.

Call chain showing the gap

PredownloadScheduler.start()
  → initializeMetricsReporter()
      → new PredownloadMetrics()       // created
      → ServerMetrics was never set up // missing — falls back to NOOP

In contrast, the normal server startup path initializes ServerMetrics via PinotMetricUtils.getPinotMetricsRegistry() before any metrics are used.

Changes

This PR adds ServerMetrics initialization to PredownloadScheduler.initializeMetricsReporter():

1. Creates a ServerConf from the existing _pinotConfig
2. Initializes PinotMetricsRegistry via PinotMetricUtils.getPinotMetricsRegistry()
3. Creates and registers a ServerMetrics instance with the proper prefix, table-level metrics config, and allowed tables

Graceful fallback

The initialization is wrapped in a try-catch(Throwable) so that if the configured PinotMetricsFactory cannot be created (e.g., because a vendor-specific metrics client is not yet available during the predownload lifecycle), the predownload process continues with the default NOOP ServerMetrics rather than crashing. This ensures the predownload container remains resilient regardless of the metrics backend availability.

Before (no ServerMetrics)

void initializeMetricsReporter() {
    _predownloadMetrics = new PredownloadMetrics();
    PredownloadStatusRecorder.registerMetrics(_predownloadMetrics);
}

After (ServerMetrics initialized with fallback)

void initializeMetricsReporter() {
    try {
        ServerConf serverConf = new ServerConf(_pinotConfig);
        PinotMetricsRegistry metricsRegistry =
            PinotMetricUtils.getPinotMetricsRegistry(serverConf.getMetricsConfig());
        ServerMetrics serverMetrics = new ServerMetrics(...);
        serverMetrics.initializeGlobalMeters();
        ServerMetrics.register(serverMetrics);
    } catch (Throwable t) {
        LOGGER.error("Failed to initialize ServerMetrics; "
            + "continuing with NOOP instance", t);
    }

    _predownloadMetrics = new PredownloadMetrics();
    PredownloadStatusRecorder.registerMetrics(_predownloadMetrics);
}

Test Plan

• Verified that the predownload container starts successfully with and without a metrics factory on the classpath
• When the metrics factory is available, ServerMetrics is properly initialized and metrics are emitted
• When the metrics factory is unavailable, the catch block logs the error and the container continues with NOOP metrics without crashing