Add nydusd_image_info metric mapping daemons to served images

[Fricounet]

Overview

Currently, there's no way to map a daemon id to an image ref using the prometheus metrics which makes debugging a bit cumbersome.
Add a new Prometheus gauge metric nydusd_image_info that maps nydus daemon IDs
to the image references they serve. For dedicated daemons this is a 1:1 mapping;
for shared daemons it is 1:N.

Related Issues

None.

Change Details

• New nydusd_image_info gauge with labels {daemon_id, image_ref}, set to 1
  for each active daemon-to-image association.
• Metric is collected in AddRafsInstance and deleted in RemoveRafsInstance.
• DestroyDaemon now calls RemoveRafsInstance for any remaining RAFS instances
  before UmountRafsInstances, covering error/forced-shutdown paths where
  individual Umount calls were skipped.

Test Results

Tested locally, metrics are added as expected and cleaned up when the image is removed:

$ curl -s http://localhost:9110/v1/metrics | grep image_info
# HELP nydusd_image_info Mapping of nydus daemon to served image references.
# TYPE nydusd_image_info gauge
nydusd_image_info{daemon_id="d7gal69jjoq0eer34190",image_ref="localhost:5000/my-image:nydus"} 1

Change Type

• Bug Fix
• Feature Addition
• Documentation Update
• Code Refactoring
• Performance Improvement
• Other (please describe)

Self-Checklist

• I have run a code style check and addressed any warnings/errors.
• I have added appropriate comments to my code (if applicable).
• I have updated the documentation (if applicable).
• I have written appropriate unit tests.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 24.42%. Comparing base (fc330cc) to head (07e2aad).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/metrics/collector/daemon.go	0.00%	4 Missing ⚠️
pkg/daemon/daemon.go	0.00%	3 Missing ⚠️
pkg/manager/manager.go	0.00%	2 Missing ⚠️
pkg/metrics/collector/collector.go	0.00%	2 Missing ⚠️
pkg/metrics/registry/registry.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #739      +/-   ##
==========================================
+ Coverage   22.02%   24.42%   +2.39%     
==========================================
  Files         130      132       +2     
  Lines       11931    12249     +318     
==========================================
+ Hits         2628     2992     +364     
+ Misses       8960     8893      -67     
- Partials      343      364      +21     

Files with missing lines	Coverage Δ
pkg/metrics/registry/registry.go	0.00% <0.00%> (ø)
pkg/manager/manager.go	0.00% <0.00%> (ø)
pkg/metrics/collector/collector.go	0.00% <0.00%> (ø)
pkg/daemon/daemon.go	8.04% <0.00%> (+8.04%)	⬆️
pkg/metrics/collector/daemon.go	0.00% <0.00%> (ø)

... and 15 files with indirect coverage changes

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