clintropolis opened a new pull request, #19632:
URL: https://github.com/apache/druid/pull/19632

   ### Description
   This PR fixes and improves metric tracking for partial segment loading, 
focusing on the `StorageMonitor`. Prior to this PR only the metadata header was 
tracked (and only partially, sometimes incorrectly).
   
   To assist, a `PartialSegmentDownloadListener` interface has been introduced 
and passed to the file mapper so that callers can be notified whenever 
operations like a download happen, which is used to wire it into the 
`StorageLocation` metrics tracking (since it is no longer 
`SegmentLocalCacheManager` driving these things when partial loads are 
involved, instead it is the caller, e.g. query engines that retrieved segments).
   
   Additionally, some new metrics:
   
   * `storage/virtual/read/count` - number of deep-storage range read 
operations.
   * `storage/virtual/read/bytes` - bytes pulled over the wire from range read 
operations.
   * `storage/virtual/read/time` - total time spent in range reads (ms)
   
   Finally, fixes flakiness in `QueryVirtualStorageTest` caused by the change 
in #19494 to not emit metrics when values are zeros, which this test was using 
as a sort of latch to indicate when activity on a StorageLocation had cleared 
out from a prior test. Instead, this is now done through new 
`tryWaitForNextEvent` and `awaitMetricQuiescent` methods on `LatchableEmitter` 
to watch for the monitor to either stabilize or stop emitting.
   


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