Add a root span to the client

[poljar]

This allows us to have a nice single tree view of the whole client run instead of having traces scattered around:

Not all methods have been handled, and for some methods we would need to apply the root as a parent only if there isn't a previous parent, but this should be a good start.

Requires: #1446

[jplatte]

This is a method that gets called from other sync methods. Using parent = here will make that relation invisible in logs, right?

[poljar]

It's called from the sync() method, which doesn't create a span itself.

[poljar]

One of the problems here is that our root span ~never gets uploaded. My comment that we can drop the span and get it emitted is false.

The spans end up in a neat tree, but the root one is missing and there are warnings about an invalid parent id.

This seems to be related: open-telemetry/opentelemetry-rust#888.

[poljar]

This PR proved that we want to have a root span, though I don't think a library should create root spans. What we instead want is to let EX create a root span and have that either be automatically propagated over the FFI or let the ClientBuilder take a root span.

Will happen in a different PR.

[codecov]

Codecov Report

Base: 74.43% // Head: 74.54% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (bef9243) compared to base (7cc06cb).
Patch coverage: 85.24% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1447      +/-   ##
==========================================
+ Coverage   74.43%   74.54%   +0.10%     
==========================================
  Files         124      124              
  Lines       13841    13881      +40     
==========================================
+ Hits        10303    10348      +45     
+ Misses       3538     3533       -5     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/sliding_sync.rs	0.00% <ø> (ø)
crates/matrix-sdk/src/room/joined.rs	65.64% <81.48%> (+2.48%)	⬆️
crates/matrix-sdk/src/client/mod.rs	88.75% <82.60%> (+0.73%)	⬆️
crates/matrix-sdk/src/client/builder.rs	76.98% <100.00%> (+1.56%)	⬆️
crates/matrix-sdk/src/client/login_builder.rs	26.35% <100.00%> (+0.57%)	⬆️
crates/matrix-sdk/src/room/timeline/mod.rs	80.34% <100.00%> (ø)
crates/matrix-sdk/src/room/timeline/inner.rs	67.95% <0.00%> (+0.11%)	⬆️
crates/matrix-sdk-crypto/src/machine.rs	76.31% <0.00%> (+0.52%)	⬆️
... and 1 more

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