feat(query): histogram_avg PromQL function

[saathvikrv]

Pull Request checklist

• The commit(s) message(s) follows the contribution guidelines ?
• Tests for the changes have been added (for bug fixes / features) ?
• Docs have been added / updated (for bug fixes / features) ?

Current behavior : (link exiting issues here : https://help.github.com/articles/basic-writing-and-formatting-syntax/#referencing-issues-and-pull-requests)

PromQL Query to calculate average from histogram time series:
sum(rate(http_request_latency::sum{_ns_="App-0",_ws_="demo"}[5m])) / sum(rate(http_request_latency::count{_ns_="App-0",_ws_="demo"}[5m]))

• MSPE executes twice, unnecessary computation
• Long query, less user friendly

New behavior :

Implemented new PromQL aggregation function - havg
New Query: histogram_avg(rate(http_request_latency{_ns_="App-0",_ws_="demo"}[5m]))

• Much more user readable
• Fetches sum and count together in the same scan to get better performance
• Adheres to official PromQL function syntax

Testing :

• Unit tests added for histogram_avg function
• Unit tests added for Double-Double schema cumulative and delta rate and increase range functions. These tests were added using Claude code
• Manual integration using 2 FiloServers in asper dev setup; Ensured that the above two queries have the same result

Benchmark Results :

[info] Benchmark                                                   Mode  Cnt    Score   Error  Units
[info] Base2ExponentialHistogramQueryBenchmark.histAvgNewQueries  thrpt    2  730.126          ops/s
[info] Base2ExponentialHistogramQueryBenchmark.histAvgOldQueries  thrpt    2  423.102          ops/s

Acknowledgments to @vishramachandran who mentored me with this project.