Add variable value instead of prometheus metrics in init_worker_by_lua_block

[SanaZulfiqar73]

Hey,

I have this scenario where I am calculating the latency value by subtracting two value and want to expose it as prometheus metrics. Is there any way I can do expose this value?
Probably something like this:

http {
    map $host $request_time_ms {
            default '';
    }

    log_by_lua '
          ngx.var.request_time_ms = $request_time - $upstream_custom_response_time
    ';

    lua_shared_dict prometheus_metrics 10M;
    lua_package_path "/path/to/nginx-lua-prometheus/?.lua;;";

    init_worker_by_lua_block {
        prometheus = require("prometheus").init("prometheus_metrics")

        metric_latency = prometheus:histogram(ngx.var.request_time_ms, "HTTP request latency", {"uri", "status"})
      }
}

[knyar]

Something like this should work:

log_by_lua '
  ngx.var.request_time_ms = ngx.var.request_time - ngx.var.upstream_custom_response_time
  metric_latency:observe(ngx.var.request_time_ms, {ngx.var.server_name, ngx.var.status})
';