Better config-time validation for overlapping flow metrics configuration

[wbutler]

Closes #2784.

This change implements a BFS walk of the processor pipeline at construction time to validate that flow metrics do not collide.

• In runtime_pipeline.rs, introduce a helper func to capture a flat edge list vector of pipeline connections and pass the result as a new param on build_flow_metric_state. Saves us from having to pass in some part of _config into that function and keeps data types better separated.
• In flow_metrics.rs:
  • In build_flow_metric_state, add a new data structure that captures start\end pairs for registered flow metrics.
  • at the end of the main loop, iff the count of flow metrics > 1
    • convert the list of edges into a one-to-many adjacency data structure
    • for each flow metric
      • do a BFS to find all the nodes between the start and the end
      • check for nodes in this set that also appear as a start node in the data structure above.
      • If found, throw an error, halting pipeline construction.

Tests

Within flow_metrics.rs, this change adds 10 unit tests, as follows:

• 7 tests exercise the interleaving detection logic on synthetically created pipeline topologies.
  • Three expected-pass tests include:
    • Linear topology with disjoint ranges
    • Branching topology with disjoint ranges
    • Linear topology with a single flow metric
  • Four expected-reject tests include:
    • Linear topology with interleaving
    • Linear topology with interleaving, reverse declaration order
    • Diamond topology with interleaving
    • Linear topology with fully nested ranges
• 3 tests exercise a helper function directly.

Validation

The following commands pass cleanly:

cargo check -p otap-df-engine
cargo test -p otap-df-engine (contains new tests)
cargo clippy -p otap-df-engine --all-targets -- -D warnings
cargo fmt --all -- --check
cargo xtask quick-check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.93%. Comparing base (f562f90) to head (d34bba0).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2983    +/-   ##
========================================
  Coverage   85.93%   85.93%            
========================================
  Files         725      725            
  Lines      275782   275980   +198     
========================================
+ Hits       236996   237172   +176     
- Misses      38262    38284    +22     
  Partials      524      524            

Components	Coverage Δ
otap-dataflow	87.06% <100.00%> (+<0.01%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	89.57% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

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[wbutler]

@drewrelmas should get a look at this since it bubbles up to his flow metrics work. Feedback & comments are very welcome.

[lalitb]

LGTM.

[wbutler]

Created #3009 to capture the follow-up hardening for unreachable end nodes.

[wbutler]

Ready for merge, IMO.