feat: add Push instrumentation

[caalador]

Add instrumentation for push.

Closes #69

[sissbruecker]

When testing this with the business-starter, I can see two spans pop up in a regular interval:

The first one is the request, the other one is created by the instrumentation and doesn't seem to run within the request. Is this intentional?

Also I tested this with an app downloaded from start.vaadin.com (running in production mode), here the only the Push: onRequest span shows up once, and then nothing else, even though push works fine 🤷‍♂️.

Maybe it would be more interesting to instrument UI.access rather than the individual handlers, although if requests already create a root span, like with the business starter, then it would be good to mark them as push requests somehow.

[caalador]

The empty span if you look into it is most likely the 101 Switching Protocols request which is handled by the server without coming to application code at all.

[sissbruecker]

Thanks, I guess we can't do anything about that one.

Still I'm not sure what the purpose of this instrumentation is. I just added some push implementation that does some SQL queries that show up as root spans, instead of getting nested under a push span. For push, the only expectation that I had was that I can see that some operation in my app was run as part of push, rather than a frontend request. Is that possible somehow?

[caalador]

I would perhaps leave this to wait for now as the idea was to get the unknown /* and / handled, but as they are mostly only 101 requests it didn't help that much. I would expect onMessage to be called when a push is sent from the server, but with all the other issues we have I didn't investigate further with changes to an application.

[caalador]

Closing PR as there is nothing that seems to help with push handling.