[Bug] Fix cluster health API response

[amkhar]

Describe the bug
Original Issue opened : #5098 . This issue is a sub task for the overall project defined #8098.

Description
Today, during node drop and joins, we fetch the metadata of unassigned shards on per shard basis. As part of the project #8098 we're changing the fetches to happen for a batch of shards. These async fetches are given in the response of _cluster/health API. We need to make changes to correctly reflect that value after fetch is happening at a batch level.

OpenSearch/server/src/main/java/org/opensearch/gateway/GatewayAllocator.java

Lines 117 to 126 in b2a7348

	public int getNumberOfInFlightFetches() {
	int count = 0;
	for (AsyncShardFetch<TransportNodesListGatewayStartedShards.NodeGatewayStartedShards> fetch : asyncFetchStarted.values()) {
	count += fetch.getNumberOfInFlightFetches();
	}
	for (AsyncShardFetch<TransportNodesListShardStoreMetadata.NodeStoreFilesMetadata> fetch : asyncFetchStore.values()) {
	count += fetch.getNumberOfInFlightFetches();
	}
	return count;
	}

Additional context
Please go through the discussion #5098 to understand the overall enhancement approach and check #8098 to see the sub tasks of overall project.