Make crates.io branch the default branch

[hds]

Starting with #998 the master branch was updated to version 0.2 for most crates and the v0.1.x branch was created to keep the "current" versions of all the tracing crates. It is from this v0.1.x branch that the crates on crates.io are released.

The way of working is to open PRs against the master branch and merge there first. The tracing maintainers then take responsibility for back-porting the changes to the v0.1.x branch to be included in the crates.io releases.

This process prioritizes the ease of getting the v0.2 crates out.

Unfortunately, it's been almost 5 years since the split, and no v0.2 crates have been released.

Note: there are some crates which have been updated to or past v0.2 such as tracing-subscriber or tracing-error, but these are all based on tracing and tracing-core v0.1 releases.

For this reason, the following group of tracing maintainers have agreed to sort of reverse what was done in #998 and instead make the v0.1.x branch (renamed to main) the default branch going forward, PRs will first be merged into this branch and then forward-ported to the v0.2.x branch (previously master).

Maintainers:

• @hds
• @davidbarsky
• @djc

Steps:

• Create main branch from v0.1.x (including CI): fix docs and enable CI on main branch #3295
• Create v0.2.x branch from master (including CI)
• Make main branch the default and protect it
• Protect v0.2.x branch
• Keep master and v0.1.x branches protected (we need to decide how best to indicate that these branches are no longer the active ones)
• Explain branch set up on master branch: chore: explain new branch set up on master branch #3298
• Explain branch set up on v0.2.x branch: chore: explain new branch set up on v0.2.x branch #3299
• Explain branch set up on v0.1.x branch: chore: fix docs and explain new branch set up on v0.1.x branch #3301
• Backport tracing-subscriber: count numbers of enters in Timings #2944

[mladedav]

Just so nothing is accidentally lost in during the switch

• fix Name / parent variant of event!()  #2983 was merged into v0.1.x but might need to be forwardported to main? (There was a question regarding that in the PR but I didn't see any answer)
• tracing-subscriber: count numbers of enters in Timings #2944 was just merged into master so we might want to backport it to the future main

[kaffarell]

Looking forward to this!

Keep master and v0.1.x branches protected (we need to decide how best to indicate that these branches are no longer the active ones)

The only thing we can do here, I suppose, is to add a big notice in the Readme.md file, similar to the one currently on the master branch.

[hds]

@mladedav I had already included #2983 into the main branch, so that's done. I've added a checklist item to backport #2944.