fix: reduce UDF metrics

[tmenjo]

chore: remove unused metrics in reduce UDF

The metric reduce_pnf_partitions_inflight (ReducePartitionsInFlight) had been no longer visited. This commit removes it.

doc: reduce_pbq_write_time that have not been documented in metrics

The metric reduce_pbq_write_time have been visited, but not documented yet. This commit adds it to the Metrics page.

chore: redefine ReduceForwardTime and ReduceProcessTime

There is no metric visited in reduce UDF meaning latency from an upstream ISB to a PBQ, nor from a PBQ to a downstream ISB. I'd say such metrics are useful to monitor pipeline's healthiness and performance.

The two metrics ReduceForwardTime and ReduceProcessTime have been defined but not visited. They look just right, so this commit redefines them. Now ReduceForwardTime means latency from an upstream ISB to a PBQ, and ReduceProcessTime from a PBQ to a downstream ISB.

fix: visit missing latency mertics in reduce UDF

This commit has reduce UDF visit the following metrics:

• forwarder_read_processing_time
• forwarder_write_processing_time
• forwarder_ack_processing_time
• reduce_data_forward_forward_time (ReduceForwardTime, redefined by the previous commit)
• reduce_pnf_process_time (ReduceProcessTime, ditto)

Close #2512

doc: two latency metrics not visited in reduce UDF

The metrics forwarder_udf_processing_time and forwarder_forward_chunk_processing_time are visited only in map UDF, not in reduce UDF. This commit states that.

[codecov]

Codecov Report

Attention: Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.

Project coverage is 71.22%. Comparing base (9b341ef) to head (48a646f).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/reduce/pnf/pnf.go	20.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2624      +/-   ##
==========================================
+ Coverage   71.11%   71.22%   +0.10%     
==========================================
  Files         397      397              
  Lines       63371    63399      +28     
==========================================
+ Hits        45066    45155      +89     
+ Misses      17190    17127      -63     
- Partials     1115     1117       +2     

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[whynowy]

We are implementing a Rust version, maybe we should continue from there? @vigith @yhl25

[vigith]

We are implementing a Rust version, maybe we should continue from there? @vigith @yhl25

If this is complete, we can merge it in. We can refer this and make sure Rust based has all the right metrics.