Stabilize the metrics feature

[Swatinem]

Removes the UNSTABLE_ prefix from the feature flag. Also runs a cargo update and updates some dependencies for good measure.

[loewenheim]

I would honestly almost split this up into two PRs—it has a number of changes that have nothing to do with the metrics feature. It should be fine either way though.

[loewenheim]

Out of interest, why is it cadence1?

[Swatinem]

probably not too relevant for the SDK because we have semver-breaking changes all the time, but the idea was to be able to support multiple versions of cadence without having to do a semver-breaking bump on our end.

[elramen]

I don't know much about the transport stuff but the changes to the normalization look good 😄

[elramen]

Is it better for performance to do multiple writes vs using collect() and join()?

Suggested change

	for (i, (k, v)) in self.tags.iter().enumerate() {
	if i > 0 {
	f.write_char(',')?;
	}
	write!(f, "{k}:{v}")?;
	}
	Ok(())
	let res = self
	.tags
	.iter()
	.map(|(k, v)| format!("{}:{}", k, v))
	.collect::<Vec<_>>()
	.join(",");
	write!(f, "{res}")

[loewenheim]

The problem with using format!, collect, and join is that they all allocate. Your version would:

1. allocate a String for each tag (the format! call)
2. collect all those strings into a newly allocated Vec (the collect call)
3. join the vector into a newly allocated String (the join call)
4. write that String into the formatter

None of these allocations are necessary. We just want to write some already existing strings into a formatter, separated by commas and colons.

[Swatinem]

The primary change here was also switching from a HashMap to a BTreeMap, to avoid having to pull in the itertools dependency to sort the map output, as that happens implicitly now.