Openresty prometheus metrics

nginx.conf

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

  metric_requests = prometheus:counter(
      "nginx_http_requests_total",
      "Number of HTTP requests",
      {"host", "status"}
  )
  metric_latency = prometheus:histogram(
      "nginx_http_request_duration_seconds",
      "HTTP request latency",
      {"host"}
  )
  metric_connections = prometheus:gauge(
      "nginx_http_connections",
      "Number of HTTP connections",
      {"state"}
  )
  metric_client_ips = prometheus:counter(
      "client_ips",
      "Number of requests by client IP",
      {"host", "ip"}
  )
  metric_user_agent = prometheus:counter(
      "nginx_http_user_agent_counts",
      "Number of requests by User-Agent",
      {"user_agent"}
  )
  metric_referer = prometheus:counter(
    "nginx_http_referer_ips",
    "Number of requests by Referer IP",
    {"ip"}
)
}

log_by_lua_block {
  metric_requests:inc(1, {ngx.var.server_name, ngx.var.status})
  metric_latency:observe(tonumber(ngx.var.request_time), {ngx.var.server_name})
  metric_client_ips:observe(1, {ngx.var.server_name, ngx.var.remote_addr})

  local user_agent = ngx.var.http_user_agent
  if user_agent then
      metric_user_agent:inc(1, {user_agent})
  end

  local referer = ngx.var.http_referer
  if referer then
      local http = require("resty.http")
      local httpc = http.new()
      local parsed_url = httpc:parse_uri(referer)
      metric_referer:inc(1, {parsed_url.host})
  end
}