Update to activeAgentCount for a Group in cloud / ops manager public API

E.g., https://docs.cloudmanager.mongodb.com/reference/api/groups/#entity-fields

The activeAgentCount description could use the following couple tweaks:

1. Instead of just Monitoring agents, it's the total number of active agents of any agent "type". So for example, if 2 monitoring agents, 1 backend agent, and 5 automation agents, this value will be 8.
2. The value is refreshed every 30 minutes, so when agents are brought down or introduced this the value won't be updated for up to 30 minutes in the worst case.