HBASE-30242 Add Region server level HFile Compression Metrics

[Umeshkumar9414]

Local testing

Note : In case of compression NONE, ratio goes above 1 becuase storeFileSize includes HFile trailer/index overhead that isn't counted in the uncompressed data blocks.

[apurtell]

Generally for compression factor, higher is better. This is what command line compression tools like xz or zstd or etc. typically report. And for compression savings, higher is also better, e.g 1 − compressed / uncompressed, which is easy to read as a percentage. This method provides the inverse of that so it makes the desired direction "down", which is awkward for Prometheus or Grafana dashboards. Operators reading a dashboard tile labeled storeFileCompressionRatio with a value e.g. of 0.33 will routinely interpret it the opposite of what is intended.

Recommendation: either rename to storeFileCompressedToUncompressedRatio so the direction is clearly established by the name of the metric, or reimplement as uncompressed / compressed, or storeFileCompressionSavingsRatio as (1 − compressed/uncompressed).

[apurtell]

A value of "1.0" will generally look identical to an operator like somehow compression has stopped working, and will generate false positive alerts.

(Claude calls this a "footgun for dashboards", lol. He means well...)

Recommendation: Return 0.0 instead, and update the description to something like "Returns 0.0 when there is no data."

[Copilot]

Pull request overview

Adds RegionServer-level metrics for total uncompressed HFile bytes and an overall store-file compression ratio, surfaced through the existing MetricsRegionServer wrapper/source plumbing and validated via unit tests.

Changes:

• Extend MetricsRegionServerWrapper with getStoreFileUncompressedSize() and getStoreFileCompressionRatio().
• Compute/aggregate uncompressed store bytes in MetricsRegionServerWrapperImpl and export both gauges via MetricsRegionServerSourceImpl.
• Update RegionServer metrics unit tests and wrapper stubs to cover the new gauges.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestMetricsRegionServer.java	Asserts the two new gauges are emitted by the metrics source.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerWrapperStub.java	Adds stub implementations returning deterministic values for the new metrics.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerWrapperImpl.java	Aggregates uncompressed bytes from stores and computes RS-level compression ratio.
hbase-hadoop-compat/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerWrapper.java	Extends wrapper interface with uncompressed size and compression ratio accessors.
hbase-hadoop-compat/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerSourceImpl.java	Exports the new wrapper values as gauges in the metrics record.
hbase-hadoop-compat/src/main/java/org/apache/hadoop/hbase/regionserver/MetricsRegionServerSource.java	Defines metric names and descriptions for the two new gauges.

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