feat: add reduce metrics for rust data plane

[adarsh0728]

What this PR does / why we need it

Metrics Added

Metric	Type	Description
reduce_active_windows	Gauge	Number of currently open reduce windows
reduce_closed_windows	Gauge	Number of closed windows awaiting GC
reduce_watermark_lag	Gauge	Difference between wall clock and watermark (ms)
reduce_window_processing_time	Histogram	Window open-to-close latency (μs)
reduce_pnf_process_time	Histogram	UDF reduce function execution time per window (μs)
reduce_pbq_write	Counter	Total messages written to PBQ

• Watermark exposed as lag, not raw epoch — wall_clock - watermark in milliseconds is directly alertable (e.g., "lag > 5min")

• Updated metrics.md as well

Related issues

Fixes #3234

Testing

Test Pipelines

• Aligned (fixed window) — examples/6-reduce-fixed-window.yaml — even-odd-sum with 60s fixed windows, 2 partitions, emptyDir storage
• Unaligned (session window) — examples/12-simple-session-pipeline.yaml — simple-session-counter with 120s session timeout, 1 partition, PVC storage

Aligned (Fixed Window) — even-odd-sum Pipeline
Total inputs sent: 101 messages (1 priming + 100 batch)

Metric	Replica 0	Replica 1	Verdict
reduce_pbq_write_total	51	50	✅ Sum = 101, exactly matches input
reduce_active_windows	1	1	✅
reduce_closed_windows	0	0	✅
reduce_watermark_lag	3703 ms	3705 ms	✅ ~3.7s lag
reduce_window_processing_time	count=1, sum=12.0M μs	(window not yet closed)	✅
reduce_pnf_process_time	count=1, sum=12.0M μs	(window not yet closed)	✅

Unaligned (Session Window) — simple-session-counter Pipeline
Total inputs sent: 101 messages (1 priming + 100 batch)
After 60s idle: counter stays at 101 — no WMB inflation.

Metric	Value	Verdict
reduce_pbq_write_total	101	✅ Exactly matches input
reduce_active_windows	2	✅ Two keyed sessions (even/odd)
reduce_closed_windows	0	✅
reduce_watermark_lag	33953 ms	✅ ~34s, matches maxDelay: 30s config
reduce_window_processing_time	not emitted	✅ Aligned-only by design
reduce_pnf_process_time	not emitted	✅ Aligned-only by design

[codecov]

Codecov Report

❌ Patch coverage is 88.14433% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.51%. Comparing base (ce2ac17) to head (4f04d15).

Files with missing lines	Patch %	Lines
...aflow-core/src/reduce/reducer/unaligned/reducer.rs	52.94%	16 Missing ⚠️
...umaflow-core/src/reduce/reducer/aligned/reducer.rs	87.50%	6 Missing ⚠️
rust/numaflow-core/src/metrics/mod.rs	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3364      +/-   ##
==========================================
- Coverage   82.53%   82.51%   -0.03%     
==========================================
  Files         307      307              
  Lines       75325    75517     +192     
==========================================
+ Hits        62172    62313     +141     
- Misses      12595    12649      +54     
+ Partials      558      555       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.