Use instrumentation classes for Service Ping generation

What does this MR do and why?

Related #352964 (closed)

Make feature merge_service_ping_instrumented_metrics default_enabled: true

Changes tested #352964 (comment 851293720)

The feature flag is enabled since 2021-02-22

With these changes, we can add instrumentation class only without any change in usage_data.rb file.

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