[DPE-6073] add requested alerts

[MiaAltieri]

Addressing #517 by adding the following requested alerts:

• Cluster is not writable
• Cluster will not be writable if I lose one more node
• Number of connections is close to max connections limit

along with a few others from the Percona alert rules

testing

• Cluster is not writable

• Cluster will not be writable if I lose one more node - note this is firing because it was deployed with a single replica, when the replica set is scaled up it goes back to green

• Number of connections is close to max connections limit (80%)