assure that index.json doesn't hold the lock too long

[Dieterbe]

e.g. /metrics/index.json - this can get very significant on multi-tenant instances ; i would be curious for a quick test actually with some fake metrics data

func (m *UnpartitionedMemoryIdx) List(orgId uint32) []idx.Archive {
	pre := time.Now()
	bc := m.RLockLow()
	defer bc.RUnlockLow("List", nil)

	defs := make([]idx.Archive, 0)
	for _, def := range m.defById {
		if def.OrgId == orgId || def.OrgId == idx.OrgIdPublic {
			defs = append(defs, CloneArchive(def))
		}
	}

	statListDuration.Value(time.Since(pre))

	return defs
}

[fkaleo]

@robert-milan to have a look and try to solve it unless the resulting code is too complex.

[stale]

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