Exports GRPC call metrics and Feast resource metrics in Core

[davidheryanto]

This pull request adds the following Prometheus metrics to Feast Core:

• feast_core_request_latency_seconds: duration of each GRPC call (labelled by service name, method name and GRPC status code)
• feast_core_feature_set_total: no of registered Feature Set
• feast_core_store_total: no of registered Store
• feast_core_max_memory_bytes
• feast_core_total_memory_bytes
• feast_core_free_memory_bytes
• feast_core_gc_collection_seconds

These metrics can be used to monitor:

• stability and performance of Feast Core, for example by monitoring the latency, traffic, error and saturation via the histogram values of feast_core_request_latency_seconds.
• usage of Feast by counting the of Feature Sets and Stores registered in Feast.

Example how Feast Core dashboard can look like:

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: davidheryanto

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[davidheryanto]

/hold
I want to export memory (and perhaps CPU) usage of the JVM too.

[davidheryanto]

/hold cancel

[zhilingc]

/lgtm