Expose Memcached Proxy Mode Stats

[Stern87]

This PR adds full support for the new proxy_* stats that appear when Memcached is built and run in proxy mode, and ensures that scraping a non-proxy Memcached instance do not fail due to missing keys. It also updates the README to document all of the new proxy metrics.

Changes:

• Exporter (exporter.go):
  • Added prometheus.Desc fields for each proxy metric (proxy_conn_requests, proxy_conn_errors, proxy_conn_oom, proxy_req_active, proxy_config_reloads, proxy_config_reload_fails, proxy_config_cron_runs, proxy_config_cron_fails, proxy_backend_total, proxy_backend_marked_bad, proxy_backend_failed, proxy_request_failed_depth, round_robin_fallback, unexpected_napi_ids).
  • Initialized those Descs in New(...) using the naming convention memcached_<metric>[_total].
  • Added those in Describe list of all the metrics exported by the memcached exporter.
  • Extended parseStats to detect and emit proxy metrics when present.
  • Wrapped proxy parsing in a guard so that missing proxy_* stats do not cause scrape failures on non-proxy Memcached instances.
• Documentation (README.md):
  • Added a Proxy Mode Metrics to the Collectors section listing and describing each exported memcached_proxy_* metric.

Motivation:

Memcached 1.6.23+ can run as a frontend proxy, exposing a new set of proxy_* stats. Without these changes, the Prometheus exporter doesn’t surface any of the proxy metrics to Prometheus.

This update makes the exporter fully compatible with both classic and proxy-mode Memcached deployments.

Testing:

• Ran the exporter against a vanilla Memcached 1.6.37: scrape succeeds, no proxy metrics.
• Ran against a proxy-mode Memcached 1.6.37 with a simple Lua routing config: all memcached_proxy_* metrics appear and values match stats output.
• Verified that existing standard metrics (uptime, cmd_get, etc.) are unaffected.

Backward Compatibility:

Existing users of the exporter who do not enable proxy mode will see identical behavior, and their scrapes will continue to succeed without any new metrics.

Documentation:

• README.md updated with adding a Proxy Mode Metrics to the Collectors section listing and describing each exported memcached_proxy_* metric.

[Stern87]

Hey @SuperQ can you take a look please?

[Stern87]

@SuperQ please review. Thanks!

[Stern87]

Hey @SuperQ can you take a look please?

[Stern87]

Hey @SuperQ can you take a look please?