Including time spent on merging shard responses on coordinating node in the profiler

[Poojita-Raj]

Is your feature request related to a problem? Please describe.
A coordinating node that receives a search request executes it in 2 parts -

1. Coordination fan-out phase: This is where the request is forwarded to data nodes that hold the data for execution.
2. Coordination fan-in phase: This is where the individual data node results are returned and merged on the coordination node for the final result.

Currently, the profiler does not take this coordination time into account when profiling a search query. The profiler is meant to be used as a diagnostic tool to understand why certain requests are slow.

This coordination time would help in providing greater clarity to the end user on what could be causing their requests to be slow.

Describe the solution you'd like
Modify the profile response to include a coordination_time field that provides the user with additional information regarding-

• coordination_fan_out : Time spent forwarding requests to other data nodes.
• coordination_fan_in: Time spent receiving individual results from other data nodes and merged into final result.

Describe alternatives you've considered
N/A

Additional context
#705