Add scaffolding for database metrics in CLI

What does this MR do and why?

Related to #464066 (closed)

Related to #569189 (closed)

Add scaffolding for database metrics in CLI

This MR sketches out the architecture to be used when creating database metrics with internal events CLI. This is just a first step towards actually adding database metrics creation to our CLI.

References

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Before	After

How to set up and validate locally

1. Make sure that old CLI database metric creation flow works as before: scripts/internal_events/cli.rb -> new metric -> database metric. This should show a message about db metrics not being supported and end the flow.
2. After checking that, restart CLI with an ENV variable: ENABLE_DATABASE_METRIC=true scripts/internal_events/cli.rb
3. Go into metric creation -> database metric again
4. Choose a description, product group and any other attributes required by the flow.
5. Accept file creation. This should create a file, filling out all the other metric fields with default values defined in this MR.
6. Make sure that the newly added metric file works correctly with Service Ping: open a new rails console and execute require_relative 'spec/support/helpers/service_ping_helpers.rb'; {'28d': ServicePingHelpers.get_current_usage_metric_value('issues_count_monthly'), '7d': ServicePingHelpers.get_current_usage_metric_value('issues_count_weekly')}. These should report values equal to the amount of issues you created in GDK in the past week/month. In case the numbers look wrong, try to comment out the cache line in the metric class reload! the rails console and re-calculate the metrics.

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