[BUG] Aggregate phase_took results for cross cluster search

[dzane17]

Describe the bug

For CCS requests, the phase_took response is retrieved from a single remote cluster SearchResponse. This is incorrect when there are 2+ remote clusters.

See getMergedResponse function

Related component

Search

To Reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

getMergedResponse should aggregate phase_took data from all remote cluster SearchResponse objects. We can use sum, max, or average to maintain the response structure. I initially thought sum is best but this will break the expectation of total phase_took times roughly equalling took time.

Additional Details

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[getsaurabh02]

Thanks @dzane17 for opening this issue. We hope you will take this forward with the PR?