docs: add Apache APISIX to software exposing Prometheus metrics

[Yilialinn]

Summary

• add Apache APISIX to the list of software exposing Prometheus metrics;
• link to the maintained Apache APISIX Prometheus plugin reference; and
• mark it as directly instrumented with a Prometheus client library.

Apache APISIX registers gateway metrics through its bundled
nginx-lua-prometheus-api7 client library and exposes them from its
first-party prometheus plugin, so no separate exporter is required. The
entry is placed in the same section as other software with directly
instrumented Prometheus endpoints.

Representative output from Apache APISIX 3.17.0:

# HELP apisix_http_latency HTTP request latency in milliseconds per service in APISIX
# TYPE apisix_http_latency histogram
apisix_http_latency_count{type="request",route="checkout-route",service="checkout",consumer="",node="172.18.0.2",request_type="traditional_http",request_llm_model="",llm_model=""} 4

# HELP apisix_http_status HTTP status codes per service in APISIX
# TYPE apisix_http_status counter
apisix_http_status{code="200",route="checkout-route",matched_uri="/status/*",matched_host="",service="checkout",consumer="",node="172.18.0.2",request_type="traditional_http",request_llm_model="",llm_model="",response_source="upstream"} 2

Validation:

• the sample was captured from a version-pinned Apache APISIX 3.17.0 Docker
  environment after requests reached a test upstream
• promtool 3.13.2 check metrics reports that the existing
  apisix_http_status counter does not use the recommended _total suffix
• apisix_http_latency reports milliseconds, while Prometheus recommends base
  units and unit suffixes for metric names

These are existing APISIX metric-convention gaps. This directory change does
not modify the metric surface or claim full compliance with Prometheus naming
guidance.

Disclosure: I am an Apache APISIX Committer.

@RichiH, could you review this small integrations-directory addition when you
have time?

[roidelapluie]

This does not seem to be direct which is for users of prometheus official libraries (which do not exist for lua).

[jan--f]

lgtm, the build failure has been addressed on main.