[I] help request: How to expose custom Prometheus metric with custom plugin [apisix]

["br41n10 (via GitHub)" <gi...@apache.org>]

br41n10 opened a new issue, #11352:
URL: https://github.com/apache/apisix/issues/11352

   ### Description
   
   How to expose a metric from custom plugin, using the default prometheus exported URI `/apisix/prometheus/metrics`.
   
   In my case, I want get summary metric about request_time, not the builtin histogram metric `apisix_http_latency_bucket`.
   
   Here is a example plugin code
   
   ``` lua
   local base_prometheus = require("prometheus")
   local core = require("apisix.core")
   local ngx       = ngx
   local clear_tab = core.table.clear
   local plugin_name = "request-time-metric"
   local prometheus = require("apisix.plugins.prometheus.exporter")
   
   local custom_metrics = {}
   
   local schema = {
       type = "object",
       properties = {},
   }
   
   local _M = {
       version = 0.1,
       priority = 40,
       name = plugin_name,
       schema = schema,
   }
   
   -- need modify ngx_tpl.lua to call this function, I don't think this is a better way
   function _M.http_init()
       if ngx.get_phase() ~= "init" and ngx.get_phase() ~= "init_worker"  then
           return
       end
   
       clear_tab(custom_metrics)
   
       prometheus = base_prometheus.init("prometheus-metrics", "apisix_")
   
       custom_metrics.test_counter = prometheus:counter("test_counter", "test", {"test"})
   end
   
   function _M.http_log(conf, ctx)
       custom_metrics.test_counter:inc(1, "test")
   end
   
   function _M.log(conf, ctx)
       -- print "null"
       core.log.warn("metrics: ", core.json.encode(custom_metrics.test_counter, true))
   end
   
   return _M
   
   ```
   
   
   Is this the right way? Or should I just modify the builtin prometheus/exporter.lua plugin (is there will be a problem when upgrade apisix?) ? 
   
   ### Environment
   
   - APISIX version (run `apisix version`): 3.9.1
   - OpenResty / Nginx version (run `openresty -V` or `nginx -V`): openresty/1.25.3.1


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Re: [I] help request: How to expose custom Prometheus metric with custom plugin [apisix]

["wklken (via GitHub)" <gi...@apache.org>]

wklken commented on issue #11352:
URL: https://github.com/apache/apisix/issues/11352#issuecomment-2421648458

   just do it in the plugin:
   
   ```lua
   local exporter = require("apisix.plugins.prometheus.exporter")
   
   local metric_api_requests_total
   
   function _M.init()
       prometheus_registry = exporter.get_prometheus()
   
       -- registers  metrics.
       metric_api_requests_total = prometheus_registry:counter(
           "api_requests_total",
           "How many HTTP requests processed, partitioned by status code, method and HTTP path.", {
               "label1",
               "label2",
               ......
           }
       )
   end
   
   
   function _M.log(conf, ctx)
       metric_api_requests_total:inc(
           1, {
               label_value_1,
               label_value_2,
               ......
           }
       )
   end
   
   ```


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