feat: return the number of shards failed (if any) in the Header proto

[girishc13]

Describe the feature

#5336 adds the feature to returns incomplete response if one or more shards are failing. This will allow the search functionality to function even if one shard is unavailable. The shard unavailability can be temporary or permanent. Its up to the client to react on this information about incomplete responses.

Your proposal

The shard failure count is returned in the Head Runtime call metadata. Propagating this information to the client using the Header proto enables the client to make decisions if the partial response can be utilised or not.

Environment

Screenshots

[jina-bot]

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