Add custom matchers for internal events

What does this MR do and why?

Adds a maintainable & usable framework for composing assertions about internal events & metrics. The MR also migrates a more complicated use-case that was previously unable to use the existing 'internal events tracking' shared example.

Composable how?

• a new metric-based matcher mirrors the behavior of the .change matcher, so assertions can be chained together.
• a new event-based matcher mirrors the behavior of the .receive matcher, but can also be tacked onto the same assertions.

Goals:

• straightforward for engineers to implement specs for events/metrics
• communicate clearly what is being tested by reading the specs
• cover all cases where internal events/metrics are being used

Related MRs:

• Improve error messages for trigger_internal_events failures
• Migrate existing internal events shared example to use new matchers

Example usage:

RSpec.describe 'Composable assertions for events & metrics' do
  it 'increments internal events metrics for compliance dashboard' do
    expect { Gitlab::InternalEvents.track_event('g_compliance_dashboard', user: user, namespace: group) }
      .to trigger_internal_event('g_compliance_dashboard').with(user: user, namespace: group).exactly(1).time
      .and increment_usage_metric('redis_hll_counters.compliance.g_compliance_dashboard_monthly').by(1)
      .and not_trigger_internal_event('web_ide_viewed')
  end


  it 'increments database-backed issue creation metrics' do
    expect { Issues::CreateService.new(container: project, current_user: user) }
      .to increment_usage_metric('usage_activity_by_stage.plan.issues')
      .and increment_usage_metric('usage_activity_by_stage_monthly.plan.issues')
  end
end

MR acceptance checklist

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Related to #450452 (closed)