--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/plugins/mod_http_openmetrics.lua	Wed Nov 24 16:03:05 2021 +0000
@@ -0,0 +1,62 @@
+-- Export statistics in OpenMetrics format
+--
+-- Copyright (C) 2014 Daurnimator
+-- Copyright (C) 2018 Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
+-- Copyright (C) 2021 Jonas Schäfer <jonas@zombofant.net>
+--
+-- This module is MIT/X11 licensed.
+
+module:set_global();
+
+local statsman = require "core.statsmanager";
+local ip = require "util.ip";
+
+local get_metric_registry = statsman.get_metric_registry;
+local collect = statsman.collect;
+
+local get_metrics;
+
+local permitted_ips = module:get_option_set("openmetrics_allow_ips", { "::1", "127.0.0.1" });
+local permitted_cidr = module:get_option_string("openmetrics_allow_cidr");
+
+local function is_permitted(request)
+	local ip_raw = request.ip;
+	if permitted_ips:contains(ip_raw) or
+	   (permitted_cidr and ip.match(ip.new_ip(ip_raw), ip.parse_cidr(permitted_cidr))) then
+		return true;
+	end
+	return false;
+end
+
+function get_metrics(event)
+	if not is_permitted(event.request) then
+		return 403; -- Forbidden
+	end
+
+	local response = event.response;
+	response.headers.content_type = "application/openmetrics-text; version=0.0.4";
+
+	if collect then
+		-- Ensure to get up-to-date samples when running in manual mode
+		collect()
+	end
+
+	local registry = get_metric_registry()
+	if registry == nil then
+		response.headers.content_type = "text/plain; charset=utf-8"
+		response.status_code = 404
+		return "No statistics provider configured\n"
+	end
+
+	return registry:render();
+end
+
+function module.add_host(module)
+	module:depends "http";
+	module:provides("http", {
+		default_path = "metrics";
+		route = {
+			GET = get_metrics;
+		};
+	});
+end

--- a/util/openmetrics.lua	Sat Nov 20 23:54:43 2021 +0100
+++ b/util/openmetrics.lua	Wed Nov 24 16:03:05 2021 +0000
@@ -25,6 +25,7 @@
 local log = require "util.logger".init("util.openmetrics");
 local new_multitable = require "util.multitable".new;
 local iter_multitable = require "util.multitable".iter;
+local t_concat, t_insert = table.concat, table.insert;
 local t_pack, t_unpack = require "util.table".pack, table.unpack or unpack; --luacheck: ignore 113/unpack
 
 -- BEGIN of Utility: "metric proxy"
@@ -52,6 +53,68 @@
 
 -- END of Utility: "metric proxy"
 
+-- BEGIN Rendering helper functions (internal)
+
+local function escape(text)
+	return text:gsub("\\", "\\\\"):gsub("\"", "\\\""):gsub("\n", "\\n");
+end
+
+local function escape_name(name)
+	return name:gsub("/", "__"):gsub("[^A-Za-z0-9_]", "_"):gsub("^[^A-Za-z_]", "_%1");
+end
+
+local function repr_help(metric, docstring)
+	docstring = docstring:gsub("\\", "\\\\"):gsub("\n", "\\n");
+	return "# HELP "..escape_name(metric).." "..docstring.."\n";
+end
+
+local function repr_unit(metric, unit)
+	if not unit then
+		unit = ""
+	else
+		unit = unit:gsub("\\", "\\\\"):gsub("\n", "\\n");
+	end
+	return "# UNIT "..escape_name(metric).." "..unit.."\n";
+end
+
+-- local allowed_types = { counter = true, gauge = true, histogram = true, summary = true, untyped = true };
+-- local allowed_types = { "counter", "gauge", "histogram", "summary", "untyped" };
+local function repr_type(metric, type_)
+	-- if not allowed_types:contains(type_) then
+	-- 	return;
+	-- end
+	return "# TYPE "..escape_name(metric).." "..type_.."\n";
+end
+
+local function repr_label(key, value)
+	return key.."=\""..escape(value).."\"";
+end
+
+local function repr_labels(labelkeys, labelvalues, extra_labels)
+	local values = {}
+	if labelkeys then
+		for i, key in ipairs(labelkeys) do
+			local value = labelvalues[i]
+			t_insert(values, repr_label(escape_name(key), escape(value)));
+		end
+	end
+	if extra_labels then
+		for key, value in pairs(extra_labels) do
+			t_insert(values, repr_label(escape_name(key), escape(value)));
+		end
+	end
+	if #values == 0 then
+		return "";
+	end
+	return "{"..t_concat(values, ",").."}";
+end
+
+local function repr_sample(metric, labelkeys, labelvalues, extra_labels, value)
+	return escape_name(metric)..repr_labels(labelkeys, labelvalues, extra_labels).." "..string.format("%.17g", value).."\n";
+end
+
+-- END Rendering helper functions (internal)
+
 local function render_histogram_le(v)
 	if v == 1/0 then
 		-- I-D-00: 4.1.2.2.1:
@@ -286,6 +349,22 @@
 	return self.families
 end
 
+function metric_registry_mt:render()
+	local answer = {};
+	for metric_family_name, metric_family in pairs(self:get_metric_families()) do
+		t_insert(answer, repr_help(metric_family_name, metric_family.description))
+		t_insert(answer, repr_unit(metric_family_name, metric_family.unit))
+		t_insert(answer, repr_type(metric_family_name, metric_family.type_))
+		for labelset, metric in metric_family:iter_metrics() do
+			for suffix, extra_labels, value in metric:iter_samples() do
+				t_insert(answer, repr_sample(metric_family_name..suffix, metric_family.label_keys, labelset, extra_labels, value))
+			end
+		end
+	end
+	t_insert(answer, "# EOF\n")
+	return t_concat(answer, "");
+end
+
 -- END of MetricRegistry implementation
 
 -- BEGIN of general helpers for implementing high-level APIs on top of OpenMetrics