subscriber: make Compact formatter not be useless

[hawkw]

Depends on #1067

Motivation

Currently, the Compact event formatter in tracing-subscriber
is...kind of bad, and nobody seems to use it. It outputs the names of
all the spans in the current context, and (inexplicably) the names of
all the fields on the current span, but not their values. This isn't
very useful.

Solution

This branch rewrites the Compact formatter to be a little less bad.
Now, we output all the fields from the current span context, but skip
span names, and we shorten the level to a single character when it's
enabled. Additionally, using the compact formatter disables targets by
default. Now, the lines are nicely short, but maybe still useful.

Example output before this change:

...and after:

[hawkw]

this PR was already approved by @davidbarsky as #1068, but I accidentally merged that into my dev branch for #1067 instead of into master (my bad). I'm just going to go ahead and merge this once CI passes, since it was already approved.

[ForsakenHarmony]

So this no longer displays the current span at all?

Any way to have this, but with span names?

[hawkw]

@ForsakenHarmony it displays the fields on all spans in the current trace. We could add span names, or the current span's name, as well. How would you like it to be formatted?

[ForsakenHarmony]

@hawkw sorry to only get back to you now, to me the current span / whole tree of spans is one of the most important parts, maybe there could be an option to configure adding them like the previously were?

[hawkw]

@hawkw sorry to only get back to you now, to me the current span / whole tree of spans is one of the most important parts, maybe there could be an option to configure adding them like the previously were?

Okay, what do you think about something like this:

With this format, do you think it's sufficiently clear which fields belong to which span? Would you prefer the spans to be printed first, the way the default formatter does?

[ForsakenHarmony]

Hmmm, yes I think I'd prefer the spans printed first, I personally don't have a big need to know where a field comes from, as it should still be relevant for the current span.

Thanks for looking into it