PrometheusMetrics - a Prometheus <tt>/metrics</tt> endpoint

Layer: L7 Application · Build flags: PC_ENABLE_STATS, PC_ENABLE_METRICS

What this example teaches

This exposes the server's runtime counters in the Prometheus text exposition format (0.0.4) so a Prometheus server can scrape the device directly: uptime, total requests, responses by status class, active connections, slot capacity, and free heap. The counters come from the built-in stats subsystem (PC_ENABLE_STATS); metrics() just renders them in Prometheus format - so this builds on top of Stats.

One call renders the scrape body:

server.on("/metrics", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.metrics(id); });

The sketch also registers a couple of ordinary routes so the counters have something to report. Point Prometheus at the device with a scrape config like ‘static_configs: [{ targets: [’<ip>'] }]`.

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_STATS=1 -DPC_ENABLE_METRICS=1" \
  --lib="." examples/L7-Application/PrometheusMetrics/PrometheusMetrics.ino

curl http://<ip>/metrics   # Prometheus text exposition format

Annotated source

The complete sketch (PrometheusMetrics.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#define PC_ENABLE_STATS 1
#define PC_ENABLE_METRICS 1
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
void setup()
{
    Serial.begin(115200);
 
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    // A couple of normal routes so the counters have something to report.
    server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "hello"); });
    server.on("/work", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "did work"); });
 
    // The Prometheus scrape endpoint (renders the stats counters).
    server.on("/metrics", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.metrics(id); });
 
    int32_t result = server.begin(80);
    if (result < 0)
        Serial.printf("begin() failed (error %d)\n", result);
    else
        Serial.println("Prometheus metrics on :80 (curl http://<ip>/metrics)");
}
 
void loop()
{
    server.handle();
}