Add Redis Metric instrumentation class

We currently have

• DatabaseMetric
• RedisHLLMetric
• GenericMetric

We should add support for RedisMetrics in instrumentation classes

Overview

classDiagram
    BaseMetric --|> DatabaseMetric
    BaseMetric --|> RedisMetric
    BaseMetric --|> RedisHLLMetric
    BaseMetric --|> GenericMetric 

This will likely require code changes similar to the following:

• Might be similar to RedisHLLMetric
• Add a new type of metric instrumentation class specific to regular Redis counters under metrics
  • class should accept options we should be able to send event names that are set in metrics definitions
  • class should have a suggested_name
• Examples of Redis counter are https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters.rb
• Add instrumentation class for one of the Redis metrics. Update metric definition YAML file, for example counts.source_code_pushes
• Update custom rails generator lib/generators/gitlab/usage_metric/usage_metric_generator.rb
• Update cop https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/rubocop-usage-data.yml#L122
• Add tests for the new metric type class
• Add test for instrumented class
• Update tests for usage_data_metrics
• Update Metrics instrumentation docs

Note: This is a proposal might be other ways to implement this metric class.

class RedisMetric < BaseMetric
  def initialize(time_frame:, options: {})
     super
     raise ArgumentError, "options events are required" unless metric_events.present?
   end

   def metric_events
     options[:events]
   end

   def suggested_name
     # ...
   end

   def value
    redis_usage_data { Gitlab::UsageDataCounters::SourceCodeCounter.read(metric_events) }
   end 
end

counts.source_code_pushes

options:
  events: [pushes]
instrumentation_class: RedisMetric