Allocation Explain API default unassigned primary

[stefnestor]

Description

👋 Would you kindly consider defaulting to a primary shard when the Allocation Explain API is ran without a request body?

E.g. 01611206 the no-request-body returned info about a replica which stated last_allocation_status: no_attempt (which made it sound like user needed to investigate software-hardware/logic setup) but attempting the same index's primary shard with an explicit request body reported no_valid_shard_copy.

AFAICT we generally assume users should resolve primaries first in all cases, so I believe it generally makes sense that IF no request body is provided our code's random shard to report on would default attempt to report on a primary.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[idegtiarenko]

I believe it is already prioritizing unassigned primaries if any:

elasticsearch/server/src/main/java/org/elasticsearch/action/admin/cluster/allocation/TransportClusterAllocationExplainAction.java

Lines 152 to 161 in 2b3cfce

	if (request.useAnyUnassignedShard()) {
	// If we can use any shard, return the first unassigned primary (if there is one) or the first unassigned replica (if not)
	for (ShardRouting unassigned : allocation.routingNodes().unassigned()) {
	if (foundShard == null || unassigned.primary()) {
	foundShard = unassigned;
	}
	if (foundShard.primary()) {
	break;
	}
	}

[idegtiarenko]

Implemented in #76220