Added dev guide for measuring Ruby blocksmetrics-measure-block

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+# Instrumenting Ruby Code
+
+GitLab Performance Monitoring allows instrumenting of custom blocks of Ruby
+code. This can be used to measure the time spent in a specific part of a larger
+chunk of code. The resulting data is written to a separate series.
+
+To start measuring a block of Ruby code you should use
+`Gitlab::Metrics.measure` and give it a name for the series to store the data
+in:
+
+```ruby
+Gitlab::Metrics.measure(:user_logins) do
+  ...
+end
+```
+
+The first argument of this method is the series name and should be plural. This
+name will be prefixed with `rails_` or `sidekiq_` depending on whether the code
+was run in the Rails application or one of the Sidekiq workers. In the
+above example the final series names would be as follows:
+
+- rails_user_logins
+- sidekiq_user_logins
+
+Series names should be plural as this keeps the naming style in line with the
+other series names.
+
+By default metrics measured using a block contain a single value, "duration",
+which contains the number of milliseconds it took to execute the block. Custom
+values can be added by passing a Hash as the 2nd argument. Custom tags can be
+added by passing a Hash as the 3rd argument. A simple example is as follows:
+
+```ruby
+Gitlab::Metrics.measure(:example_series, { number: 10 }, { class: self.class.to_s }) do
+  ...
+end
+```