DAT-1735: Upgrading prometheus_exporter

[cajun]

What? Why?

This PR adds a Content-Type header to work with newer versions of Prometheus.

This link describes the details of the Content-Type header settings
https://prometheus.io/docs/instrumenting/content_negotiation/

How was it tested?

First I started with running two versions of Prometheus with the same scrape configuration:

docker-compose.yml

services:
  prometheus-old:
    image: prom/prometheus:v2.50.0
    volumes:
      - "./prometheus.yml:/etc/prometheus/prometheus.yml"
    ports:
      - 9090:9090
  prometheus-new:
    image: prom/prometheus
    volumes:
      - "./prometheus.yml:/etc/prometheus/prometheus.yml"
    ports:
      - 9091:9090

prometheus.yml

scrape_configs:
  - job_name: 'bc-prometheus-ruby'
    metrics_path: '/metrics'
    scrape_interval: 5s
    static_configs:
      - targets: ['host.docker.internal:9394']

Then i used the default code from the readme of this repository.

#!/usr/bin/env ruby

ENV['BUNDLE_GEMFILE'] ||= File.expand_path('Gemfile', __dir__)
require 'bundler/setup' # Set up gems listed in the Gemfile.

require 'bigcommerce/prometheus'

class AppTypeCollector < ::Bigcommerce::Prometheus::TypeCollectors::Base
  def build_metrics
    {
      honks: PrometheusExporter::Metric::Counter.new('honks', 'Running counter of honks'),
      points: PrometheusExporter::Metric::Gauge.new('points', 'Current amount of points')
    }
  end

  def collect_metrics(data:, labels: {})
    metric(:points).observe(data.fetch('points', 0))
    metric(:honks).observe(1, labels) if data.fetch('honks', 0).to_i.positive?
  end
end

class AppCollector < ::Bigcommerce::Prometheus::Collectors::Base
  def honk!
    push(
      honks: 1,
      custom_labels: {
        volume: 'loud'
      }
    )
  end

  def collect(metrics)
    metrics[:points] = rand(1..100)
    metrics
  end
end

::Bigcommerce::Prometheus.configure do |c|
  c.web_collectors = [AppCollector]
  c.web_type_collectors = [AppTypeCollector.new]
  c.resque_collectors = [AppCollector]
  c.resque_type_collectors = [AppTypeCollector.new]
  c.hutch_collectors = [AppCollector]
  c.hutch_type_collectors = [AppTypeCollector.new]
end

server = ::Bigcommerce::Prometheus::Server.new
Bigcommerce::Prometheus.web_type_collectors.each do |tc|
  server.add_type_collector(tc)
end

server.start_until_stopped do
  app_collector = AppCollector.new
  app_collector.honk!
  puts 'waiting'
end

Here are the metrics that are produced from the code above with the content-type being set

❯ curl -v localhost:9394/metrics
* Host localhost:9394 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:9394...
* connect to ::1 port 9394 from ::1 port 51494 failed: Connection refused
*   Trying 127.0.0.1:9394...
* Connected to localhost (127.0.0.1) port 9394
> GET /metrics HTTP/1.1
> Host: localhost:9394
> User-Agent: curl/8.7.1
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< content-type: text/plain; charset=utf-8
< content-length: 898
<
# HELP ruby_collector_metrics_total Total metrics processed by exporter.
# TYPE ruby_collector_metrics_total counter
ruby_collector_metrics_total 1


# HELP ruby_collector_sessions_total Total send_metric sessions processed by exporter.
# TYPE ruby_collector_sessions_total counter
ruby_collector_sessions_total 1


# HELP ruby_collector_bad_metrics_total Total mis-handled metrics by collector.
# TYPE ruby_collector_bad_metrics_total counter



# HELP ruby_collector_working Is the main process collector able to collect metrics
# TYPE ruby_collector_working gauge
ruby_collector_working 1


# HELP ruby_collector_rss total memory used by collector process
# TYPE ruby_collector_rss gauge
ruby_collector_rss 60512

# HELP ruby_honks Running counter of honks
# TYPE ruby_honks counter
ruby_honks{volume="loud"} 1

# HELP ruby_points Current amount of points
# TYPE ruby_points gauge
ruby_points 0
* Connection #0 to host localhost left intact

Testing Prometheus v2.50.0:

Testing Prometheus v3.10.0:

---

Note

Low Risk
Low risk: only adds an explicit Content-Type header on the /metrics endpoint and a small spec to lock in the behavior; no changes to metric generation or request routing.

Overview
Ensures the Puma /metrics endpoint always returns Content-Type: text/plain; charset=utf-8, improving compatibility with newer Prometheus content negotiation expectations.

Adds a focused Rack app spec asserting the header is present for GET /metrics responses.

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