Ruby continuous profiling support (platformsh#4137)

Co-authored-by: Chad Carlson <[email protected]>

sites/upsun/src/increase-observability/application-metrics/ruby.md

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+---
+title: Continuous profiling for Ruby
+sidebarTitle: "Ruby continuous profiler"
+description: Configure and use the Ruby continuous profiler.
+weight: 30
+---
+
+{{< vendor/name >}} [Continuous Profiler](./cont-prof.md) is powered by [Blackfire](../../../increase-observability/application-metrics/blackfire.md).
+It is available directly the [Console](/administration/web/_index.md), under the **Profiling** tab of your environments.
+
+The Ruby continuous profiling is currently made across 3 dimensions:
+- **CPU Time**:  Time spent running on the CPU
+- **Wall-time**: elapsed time per function call
+- **Allocations**: Time spent running on the CPU
+
+The default sampling frequency is 100 Hz. This means the Ruby continuous profiler is
+collecting information 100 times per second.
+
+## Prerequisites
+
+{{< vendor/name >}} Continuous Profiler requires [`Ruby >= 2.5`](/languages/ruby.md).
+
+## Installation
+
+1. Add the `datadog` gem to your `Gemfile` or `gems.rb` file:
+
+``` bash
+gem 'datadog', '~> 2.0'
+```
+
+2. Install the gems running the `bundle install` command.
+
+3. Add the ``ddprofrb exec`` command to your Ruby application start command:
+
+``` bash
+bundle exec ddprofrb exec ruby myrubyapp.rb
+```
+
+Rails example:
+``` bash
+bundle exec ddprofrb exec bin/rails s
+```
+
+Alternatively, start the profiler by adding the following code in your application's entry point if you can't start it using `ddprofrb exec`:
+
+``` bash
+require 'datadog/profiling/preload'
+```