Hey. Can I take up this issue? I have recently started using druid in one of my project and really liked the product. I am new to open source so don't want to pick something big at this stage.

Hi @Sarvesh1990, definitely!

For the first thing (number of merge buffers allocated), that should probably be the number of merge buffers outstanding at a point in time. For an example of another metric that does this, check jetty/numOpenConnections in JettyServerModule. And then consider adding something similar to DruidProcessingModule.

For the second thing (amount of time spent waiting for a merge buffer), that should probably be a query metric, so the thing to do would be to add a new method like reportMergeBufferWaitTime(long timeNs) to QueryMetrics. And then call it with the amount of time it took to call mergeBufferPool.take() in GroupByMergingQueryRunnerV2.

These could be done as separate PRs since they are not related.

Hey @gianm

Sorry for delay. Had to go out of station for some urgent work.
Please find PR for Point 1 : #5541

Let me know if this works. Will start working on Point 2 now.

No worries @Sarvesh1990. I left a review on your PR.

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@gianm Can you re-open the issue? I think this is still an important feature to have.