Standardize on span naming

[seanmonstar]

The prototype currently just looks for a span named tokio::task::spawn, but if want to work for any "runtime"-like thingy, we probably should have a more generic name. It still needs to be unique enough that a user would be very unlikely to create a span with the same name.

Besides task spawning, we also need to do this for wakers and resources too.

[hawkw]

I think we should probably care about just a standardized span name, rather than a standardized span name and tracing target. I think it would be good to allow runtimes to have the target differ,

For example, @nikomatsakis mentioned wanting to instrument rayon with the console. So, if you're using tokio and rayon in the same program, we could distinguish between spans named spawn with the tokio::task target, and spans named spawn with the rayon target. Although, on the other hand, we could use a custom target but also display the module path.

The other thing is whether spawn is the correct name. We may want to use something less generic, to ensure that people don't accidentally create spans with the spawn name that aren't actually supposed to correspond to tasks.

Perhaps one option would be to require overridden targets beginning with console, as well as the spawn span naming? I dunno, just throwing out ideas here.

[carllerche]

To help avoid name collision, we can probably include a prefix. e.g. "runtime.spawn".

[hawkw]

+1 for @carllerche's suggestion of runtime., that seems like a good naming convention to indicate that the span is provided by the runtime's instrumentation rather than user code, and avoid colliding with user spans, but without tying the naming scheme to an individual runtime implementation.

[carllerche]

Ref: tokio-rs/tokio#3954 with regards to figuring out how to name resource spans. My guess is we need a "kind" annotation to identify resource spans vs. operation spans as well.

[hawkw]

I believe #68 completed the console side work for this. We still need to update tokio to use the new naming scheme, though.