Easy Metrics (rb)

This project provides a facility for instrumenting applications with Prometheus metrics.

Usage

Examples TL;DR

Add the gem to your project:

  bundle add emrb

Basic usage

require "emrb"

class Metrics
  # Emrb::Instruments provides all necessary methods for creating instruments.
  include Emrb::Instruments  
  # Instruments can be defined as the following:
  counter :my_counter
end

# Perform measurements.
Metrics.my_counter.inc

Features

Supported metrics

All Prometheus metrics are supported and can be created the same way as in the example above.

Providing metric-specific configs

Most of the provided methods are nothing but a shorthand for creating instruments (with some facilities):

require "emrb"

class Metrics
  include Emrb::Instruments
  #                                          /-> All other options for each metric
  #                                          |   follow the original client's syntax.
  counter :my_counter, "this is my counter", labels: [:my_label]
  #   _________________/
  #   \_> This is the "docstring" value used for creating 
  #       metrics using the Prometheus client. 
  #       This is the only API-breaking option
  #       for enhancing usability. Defaults to "..." when absent. 
end

When your metric requires multiple options, a single line may become unwieldy. In such cases, you can use a block to define the metric config:

require "emrb"

class Metrics
  include Emrb::Instruments

  # The block should return a hash containing all the desired options.
  # When a block is used, other options passed inline are ignored.
  histogram :my_histogram do
    {
      labels: [:label1, :label2],
      preset_labels: { label1: "bar" },  
      buckets: [0.9, 1.0, ...]
    }
  end
end

Presets

If more than one of your metrics leverages the same preset_labels, the following is possible:

require "emrb"

class Metrics
  include Emrb::Instruments

  # Apply preset labels to multiple metrics within a block.
  with_presets label_a: "value_a", label_b: "value_b" do
    counter :counter_a
    counter :counter_b, labels: [:label_c]
    #                    \_> You can continue configuring 
    #                        each metric as usual.
    #                        The preset labels will be merged
    #                        with the specific ones.
  end
end
# Metrics.counter_a.labels => [:label_a, :label_b]
# Metrics.counter_b.labels => [:label_a, :label_b, :label_c]

Subsystems

You can organize your metrics into subsystems, allowing different components of your application to group related metrics:

require "emrb"

class Metrics
  include Emrb::Instruments
  subsystem :http do
    histogram :request_duration, "duration of requests" do
      { labels: [:method, :path] }
    end
  end

  subsystem :postgres do
    #                    / -> Subsystems also accepts presets.
    #                   |
    subsystem :replica, op: "read" do
      counter :op_count
    end

    subsystem :master do
      counter :op_count, labels: [:op]
    end
  end
end

Metrics within a subsystem are scoped by the subsystem's name. To access metrics within a subsystem:

Metrics.postgres.master.op_count

Subsystem's metrics name

Metrics created within subsystems are prefixed with the subsystem's name. For example:

• The request_duration metric within the http subsystem will be named http_request_duration
• The op_count metric of postgres.master will be named postgres_master_op_count.

Exposing metrics

Both Prometheus::Middleware::Exporter and Prometheus::Middleware::Collector are Rack middlewares included in this gem as Emrb::Exporter and Emrb::Collector.

If you're unfamiliar with how to use these middlewares, the http example provides a demonstration.

Pushing metrics

require "emrb"

class Metrics
  include Emrb::Instruments  
  counter :my_counter
end

# Perform measurements.
Metrics.my_counter.inc

# Push the current registry state to your gateway.
Metrics.push("example", gateway: "http://localhost:9091")

License

The MIT License (MIT)

Copyright (c) 2024 Felipe Mariotti, Vito Sartori

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.