Detect exemplars disabled for a tenant in querier, save time

[bboreham]

Exemplars can be disabled for a tenant, by setting -ingester.max-global-exemplars-per-user to 0 (this is the default). (They can be enabled per-tenant by setting an override of max_global_exemplars_per_user)

When an exemplars request comes in, we pass it on to ingesters, even if exemplars are disabled for that tenant. We could examine that setting in the querier and return immediately if it is 0.

Related: #697