Introduce a gauge metric for the spire-server entry cache size

[stevend-uber]

It would be valuable for spire users to be able to easily query how large in-memory cache size is at a metric's glance over time.

[edwbuck]

What units are you expecting? (size in number of entries, size in bytes / kb, etc.)

[azdagron]

Seems very reasonable to add a bunch of metrics around the internal cache details.

[stevend-uber]

Off the top of my head:

• Entry Counts
• Size in bytes

[tburko]

one could build an alert based on the difference between actual entries and expected and hopefully get a heads up before rotations start to fail en masse

[edwbuck]

Let's extend this to include missing event gauge

[stevend-uber]

@azdagron You can assign this issue to me. I think I can get to this in July :)

[azdagron]

Super! Thanks, @stevend-uber !