extract the metrics from rawServiceView for call_count for service entities

[kotharironak]

As part of Phase1, the existing metrics - call count/error count/latency - mainly for service, backend, and API entities first, will be extracted at the end of the ingestion pipeline after view generation. Later, these metrics will be collected by the client with the necessary metadata and ingested.

As par to this ticket, build a metrics-generator from trace data from generated views. This will involve,

• Adding new module called hypertrace-metrics-generator which reads data from rawSericeView #298
• Add the logic for SUM aggregation for num_calls #299