Have Crossplane emit prometheus metrics about function runs

[negz]

What problem are you facing?

We promoted composition functions to beta in Crossplane v1.14. I'd like to get them to v1/GA in v1.15. Right now Crossplane doesn't emit any specific metrics for functions, so would be difficult or impossible to monitor them.

How could Crossplane help solve your problem?

We should identify and add prometheus metrics to Crossplane to allow people to monitor functions.

[negz]

Note that this would be gRPC client metrics for functions, since Crossplane is a client of a function. Adding metrics to the functions themselves is tracked in #5002.