Benchmark for https://github.com/knyar/nginx-lua-prometheus/pull/82, just place both files in benchmark/ directory, then execute run.sh.

nginx.conf

daemon off;
user www-data;
pid /var/run/nginx.pid;
worker_processes 4;
include /etc/nginx/modules-enabled/*.conf;
events {}

http {
    sendfile on;
    tcp_nodelay on;

    lua_shared_dict prometheus_metrics 20M;
    lua_package_path "/nginx-lua-prometheus/?.lua;;";

    error_log stderr info;

    init_by_lua_block {
        prometheus = require("prometheus").init("prometheus_metrics")
        my_counter = prometheus:counter("counter1", "Benchmark counter", {"n"})
    }
    init_worker_by_lua_block {
        prometheus:init_worker(1)

        local get_keys_samples = 100
        local metric_data_samples = 10
        local max_metric_count = 50000
        local step = 10000

        local start_time
        local get_keys_time
        local metric_data_time

        function measure(i)
            ngx.update_time()
            start_time = ngx.now()
            for j = 1, get_keys_samples do
                -- original: prometheus.dict:get_keys(0)
                -- key_index: prometheus.key_index:get()
            end
            ngx.update_time()
            get_keys_time = (ngx.now() - start_time) / get_keys_samples

            ngx.update_time()
            start_time = ngx.now()
            for j = 1, metric_data_samples do
                prometheus:metric_data()
            end
            ngx.update_time()
            metric_data_time = (ngx.now() - start_time) / metric_data_samples

            ngx.log(ngx.INFO, i, "\t", get_keys_time, "\t", metric_data_time)
        end

        function benchmark()
            -- original: ngx.log(ngx.INFO, "Metric count\tavg. get_keys()\t avg. metric_data()")
            -- key_index: ngx.log(ngx.INFO, "Metric count\tavg. key_index:get()\t avg. metric_data()")
            for i = 0, max_metric_count do
                if i % step == 0 then
                    measure(i)
                end
                my_counter:inc(1, {i})
            end
            ngx.log(ngx.INFO, "Done, press Ctrl+C to exit.")
        end

        -- only run this on one of the workers
        if ngx.worker.id() == 0 then
            -- launch from timer, so other workers can initialize first
            ngx.timer.at(1, benchmark)
        end

    }

    server {
        listen 18001;
        server_name benchmark;
        location / {
            return 200;
        }
    }
}

run.sh

#!/bin/bash

set -x -e -u -o pipefail

base_dir="$(cd "$(dirname "$0")"; pwd -P)"
container_name="nginx_lua_prometheus_benchmark"
image_name="${container_name}_image"

cat > ${base_dir}/Dockerfile <<EOF
FROM debian:stable-slim
RUN apt-get update
RUN apt-get install --no-install-recommends -y nginx-light libnginx-mod-http-lua
EOF

docker build -t ${image_name} ${base_dir}

function cleanup {
  docker rm -f ${container_name} || true
}
cleanup
trap cleanup EXIT

if [ -f ${base_dir}/../key_index.lua ]; then
    version="key_index"
else
    version=original
fi
conf="nginx-${version}.conf"
sed "s/-- $version: //;" ${base_dir}/nginx.conf > ${base_dir}/${conf}

exec docker run -it --name ${container_name} \
  -v ${base_dir}/../:/nginx-lua-prometheus ${image_name} \
  nginx -c /nginx-lua-prometheus/benchmark/${conf}