feat: address metrics improvements

[ElastiJAM]

feat(metrics): refine metrics naming and organization

Changes

Refactors metrics naming to use consistent subsystem prefixes and standardizes processing latency metric labels. Replaces EXPORT_LATENCY_SECONDS with unified PROCESSING_LATENCY_SECONDS histogram.

Key Changes

• New ProcessingStage enum: Type-safe labels for processing latency metrics (ebpf_ringbuf_output, producer_output, decorator_output)
• Metrics reorganization: All metrics now use subsystem prefixes (e.g., mermin_ebpf_map_entries → mermin_ebpf_map_entries)
• Unified latency tracking: Replaced EXPORT_LATENCY_SECONDS with PROCESSING_LATENCY_SECONDS using decorator_output stage label
• Removed ExportStatus::Attempted: Simplified export status tracking
• Renamed metrics: FLOW_PRODUCER_FLOW_SPANS_TOTAL → PROCESSING_TOTAL for clarity
• Updated documentation: Reflects new metric names and label values

Type of change

• New feature
• Bug fix
• Breaking change
• Documentation
• Other:

Testing

• Verified metrics registry initialization with debug enabled/disabled
• Confirmed processing latency timers use correct stage labels
• Validated metric name changes don't break existing dashboards (documentation updated)

Proof it works

• All metrics compile and register correctly
• Processing latency tracked at all three stages (ebpf_ringbuf_output, producer_output, decorator_output)
• Documentation updated to reflect new metric names

Checklist

• I've tested my changes
• I've updated relevant documentation
• My code follows the project's style (run cargo fmt and cargo clippy)
• All tests pass

Notes

This is a metrics naming refinement that improves consistency and maintainability. The subsystem-based naming makes it easier to understand metric relationships and reduces naming conflicts. The unified processing latency metric provides better observability across the pipeline.

[maksym-iv-ef]

Added few notes

[maksym-iv-ef]

In a follow-up PRs we are going to iterate over unifying metrics for the eBPF maps so each eBPF map to have same subset of metrics in order to simplify eBPF map observability