Add additional metrics

[Neverlord]

To allow users to better understand what Broker is doing and to understand the health of the system, we need additional metrics. The list we've came up with so far:

• Unsent messages, i.e., number of buffered messages at the core actor
• Unsent bytes, i.e., accumulated sizes of buffered messages
• Input and output channels of stores (a master will have one channel per clone)
• Number of entries in the data store
• Number of buffered commands, i.e., command messages that wait for one or more ACKs

Ideas for metrics that would need some extra clarification or are not straightforward to implement:

• High-water metrics, e.g., largest message yet / highest amount of buffered messages?
• Latencies: this one is tricky, because ideally this time would be measured from creating an event (in Zeek) to writing the message to a socket; however, messages currently have no timestamps