Add expvar handler to zpages extension

[yurishkuro]

Is your feature request related to a problem? Please describe.

Go stdlib's expvar provides some useful information about Go runtime, and sometimes it is used by various libraries that could be imported by OTEL components to report their own internal state. But the HTTP handler is not exposed by default so there is no way to introspect that state of the libraries in the collector.

Describe the solution you'd like

Since zpage extension is the agreed mechanism for introspection of the running collector, and already provides a webserver, we can register expvar handler on that server (also mentioned in #2155 (comment)).

The zpage extension config can have a flag that enables expvar, which could be off by default.

Describe alternatives you've considered
There could be a separate expvar extension just to create a new webserver and open a port, but it seems like an overkill and not aligned with zpages as the main introspection facility.

[HongChenTW]

Hi @yurishkuro, I'd like to try this one, it seems a good opportunity to understand the zpage extension implementation.

[yurishkuro]

@HongChenTW go for it. I temporarily created an internal extension in Jaeger as a workaround jaegertracing/jaeger#5986

[HongChenTW]

Hi @yurishkuro , after digging into code, I found that I could register the expvar handler to expose the expvar pages at here, with some adjustments(declare the expvarz path variable, add port, on/off configs, etc.), but I'm not sure if I'm in the right path.

e.g. zPagesMux.Handle(path.Join("/debug", "expvarz"), expvar.Handler())

But I also noticed other components in zpages extention draw html pages like tracez or featurez, should I also design a html template to render the expvar, could you give some advises?

Thanks!

[yurishkuro]

@HongChenTW I would start with just the default, out of the box expvar.Handler(). It returns JSON which is already pretty readable, and there are probably tools today that can scrape that format if someone needs it. I personally just used a browser plugin for JSON rendering so that I can expand/collapse some of the more complex structures supported by the expvar package.