Add more XDS operational metrics

[howardjohn]

• Number of messages
• Total message size (counter) by type

[MorrisLaw]

Where are these currently defined (the XDS operational metrics)?

context: if I wanted to add this I'm not sure where to start.

[howardjohn]

src/xds/metrics.rs is what we have currently

[MorrisLaw]

Is message an existing type somewhere? Or, does it need to be created (like how ConnectionTermination is in metrics.rs) and should it have any particular fields?

[howardjohn]

I think we would just report based on

ztunnel/src/xds/client.rs

Line 672 in f18be9c

size = response.resources.len(),

.

Note size on that line is actually count. I think we also want the byte size. Which would be raw.resource.unwrap().value.len() on each responses.resources.