feat: add comprehensive git sync metrics for v2

[markphelps]

Summary

Implements comprehensive git synchronization metrics for Flipt v2, providing visibility into git-based flag synchronization operations.

Addresses: https://github.com/orgs/flipt-io/discussions/4556

New Metrics

This PR adds the following Prometheus metrics to track git sync operations:

Metric	Type	Description	Labels
flipt_git_sync_total	Counter	Total sync operations	remote, branch, status
flipt_git_sync_duration	Histogram	Sync operation latency (ms)	remote, branch
flipt_git_sync_errors_total	Counter	Categorized sync errors	remote, branch, error_type
flipt_git_files_changed_total	Counter	Files changed during sync	remote, branch, operation

Error Types

• auth - Authentication/authorization failures
• network - Network/repository access issues
• conflict - Merge conflicts or repository state issues
• repository - Repository structure problems
• unknown - Unrecognized errors

File Operations

• added - New files created during sync
• modified - Existing files changed during sync
• deleted - Files removed during sync

Implementation Details

Core Changes

• Enhanced repoMetrics struct with new sync-specific counters and histograms
• Modified Fetch() method to track sync operations and detect file changes
• Added categorizeError() function using go-git v6 error types with errors.Is()
• Added trackSyncChanges() to analyze commit diffs and count file modifications

Key Features

• ✅ Proper error categorization using go-git exported error types instead of string matching
• ✅ Change detection that analyzes git tree diffs to track file modifications
• ✅ Branch-aware metrics with branch and remote labels for environment tracking
• ✅ Non-breaking implementation that extends existing metrics without disruption
• ✅ Memory efficient using counters/histograms rather than problematic gauge metrics

Bonus Fix

Also fixes the mage formatting task to exclude all hidden directories (starting with .) instead of hardcoding specific ones like .git and .devenv. This prevents formatting issues with various hidden directories.

Example Usage

Users can now create Prometheus alerts and dashboards:

# Alert on sync failures
- alert: FliptSyncFailures
  expr: rate(flipt_git_sync_errors_total[5m]) > 0.1
  for: 5m
  annotations:
    summary: "High git sync failure rate for {{ $labels.remote }}/{{ $labels.branch }}"

# Alert on slow sync operations  
- alert: FliptSyncSlow
  expr: histogram_quantile(0.95, flipt_git_sync_duration) > 30000
  for: 5m
  annotations:
    summary: "Git sync operations are slow for {{ $labels.remote }}/{{ $labels.branch }}"

Test Plan

• Unit tests pass for new metrics functionality
• All existing git storage tests continue to pass
• Code compiles and builds successfully
• Mage formatting and linting pass
• Manual testing with actual git repository sync operations
• Verify metrics appear correctly in Prometheus/monitoring

Breaking Changes

None - this is an additive feature that extends the existing metrics system.

Documentation

The new metrics will be documented in the monitoring/observability documentation as part of the existing git storage metrics section.

[codecov]

Codecov Report

❌ Patch coverage is 39.72603% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.32%. Comparing base (6c44f8f) to head (86630f1).
⚠️ Report is 2 commits behind head on v2.

Files with missing lines	Patch %	Lines
internal/storage/git/repository.go	11.11%	83 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #4587      +/-   ##
==========================================
- Coverage   64.80%   60.32%   -4.48%     
==========================================
  Files         131      131              
  Lines       14776    14920     +144     
==========================================
- Hits         9576     9001     -575     
- Misses       4549     5276     +727     
+ Partials      651      643       -8     

Flag	Coverage Δ
integrationtests	20.22% <11.64%> (-18.20%)	⬇️
unittests	54.39% <39.72%> (-0.12%)	⬇️

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