Send usage metrics from operator proxies with default anonymous ID

[lujunsan]

PR description:

Problem

Operator-deployed MCP server proxies were not sending tool count metrics to the usage API, despite:

• The usage metrics middleware being properly configured and active
• Periodic 15-minute flushes working correctly for local deployments
• The operator controller itself successfully sending metrics

Root Cause

Operator-deployed proxies run in distroless containers with read-only root filesystems and cannot read the ~/.local/share/toolhive/updates.json file where anonymous IDs are stored.

The flow was:

1. TryGetAnonymousID() attempts to read the file but it doesn't exist
2. Returns empty string (doesn't generate ID - read-only operation)
3. Empty string gets cached in NewClient() at startup
4. SendMetrics() sees empty anonymous_id and returns nil (skips sending)
5. Metrics never reach the API

Solution

Modified pkg/usagemetrics/client.go to detect operator environments using Kubernetes environment variables. When running in K8s and anonymous_id is empty, use the default value "operator-proxy" instead of skipping the send.

This allows operator-deployed proxies to send metrics with a consistent identifier while local deployments still wait for the version check to create the anonymous ID file.

This also means we lose traceability of operator anonymous_ids for usage metrics; anonymous_id isn't a mandatory field to understand usage metrics, but can still be useful. A better solution might exist, but this unblocks usage metrics gathering for operator.

Testing

Tested locally with Kind cluster:

1. Deployed operator and fetch server
2. Made tool calls via Cursor
3. Verified metrics appeared in update service with anonymous_id=operator-proxy
4. Status 200 responses confirmed successful delivery

[lujunsan]

@ChrisJBurns would appreciate a validation and look into this one, can you think of a better solution that isn't overly complex?

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.45%. Comparing base (63c8c63) to head (d7cbb74).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/usagemetrics/client.go	62.50%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2635      +/-   ##
==========================================
- Coverage   55.47%   55.45%   -0.03%     
==========================================
  Files         312      312              
  Lines       29714    29717       +3     
==========================================
- Hits        16485    16479       -6     
- Misses      11789    11801      +12     
+ Partials     1440     1437       -3     

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