Expose more data and stats via JMX

JMX monitoring was added in 1.16, but is currently very minimal: it currently only offers tick times. This data is of course useful to have, but it would be more useful if one could correlate the data with other statistics, some of which is already being offered to players via the F3 debug menu.

I think it would be very useful to have these attributes available via JMX:

• Number of players online
• Number of chunks loaded
• Memory usage (heap size, usage etc)
• How often the GC is run (or something?)
• How many entities are loaded (debug screen even offers different categories)
• Current render distance
• Current version
• Current protocol version
• Current network latency to players
• TX and RX to players
• Current ingame time (time query gametime)
• Current ingame weather
• Current ingame moon phase
• Heck, even a cute splash screen message would be fun :)