Stricter validation of metric definitions

What does this MR do and why?

Related to #416666 (closed)

We want to make metrics validation more strict. To make sure schema.json will not be growing indefinitely, we also want to split the rules into separate files.

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Before	After

How to set up and validate locally

1. Try to create/update a metric into something that doesn't pass the new validations:
   • a metric with internal_events data_source [eg. update this metric] and any of:
     • instrumentation_class undefined or has value other than RedisHLLMetric or TotalCountMetric
     • instrumentation_class is RedisHLLMetric and options not being exactly an object with an events property that is an array of strings
     • instrumentation_class is RedisHLLMetric and events not being an array of objects having exactly name and unique string properties
     • instrumentation_class is RedisHLLMetric and events[x].unique having a value other than "user.id", "project.id", or "namespace.id"
   • a metric with redis_hll data_source [eg. update this metric] and any of:
     • instrumentation_class other than RedisHLLMetric or AggregatedMetric
     • instrumentation_class as RedisHLLMetric but options is not exactly an object with events property that has an array of strings
   • a metric with redis data_source [eg. update this metric] and any of:
     • instrumentation_class other than RedisMetric or MergeRequestWidgetExtensionMetric
     • instrumentation_class set to MergeRequestWidgetExtensionMetric but options is not exactly an object with event and widget string properties
     • instrumentation_class set to RedisMetric but options is not exactly an object with event, prefix and optional include_usage_prefix properties
2. Run a test file, for example bundle exec rspec spec/lib/gitlab/usage/metric_definition_spec.rb.
3. This should raise an error due to invalid metrics: Gitlab::Usage::MetricDefinition::InvalidError

MR acceptance checklist

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