refactor tracing

[little-dude]

use of the Traced struct has two downsides:

• Because we want to decouple the tracing logic from the application
  logic, we introduced generics all over the place for our code to
  work both with T and Traced.
• We currently pass the Traced.span() down all the services chain, but
  Spans are more of less immutable and declared in macros at compile
  time, so just passing this span down is useless: we cannot at any
  field to it.

This refactoring is twofold:

1. Remove all the genericity around tracing. This resulted in a lot of
   simplifications, especially in the state machine, but also in the
   pet message services since we could get rid of the
   _PetMessageHandler trait.

2. Replace Traced by a Request that is going to be used
   everywhere. Note that we currently just use Request in for the
   PET message services and the state machine, but it should be
   straightforward to use anywhere else. Unlike Traced<T>,
   Request<T> allows us to enrich the span attached to the request:
   Request::map creates a child span a the request is mapped.

[codecov]

Codecov Report

Merging #492 into master will increase coverage by 1.99%.
The diff coverage is 52.68%.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
+ Coverage   51.56%   53.55%   +1.99%     
==========================================
  Files          63       63              
  Lines        3105     3049      -56     
==========================================
+ Hits         1601     1633      +32     
+ Misses       1504     1416      -88     

Impacted Files	Coverage Δ
rust/src/client/request.rs	9.63% <0.00%> (ø)
rust/src/message/message.rs	39.28% <0.00%> (-6.55%)	⬇️
rust/src/rest.rs	0.00% <ø> (ø)
rust/src/services/messages/mod.rs	0.00% <0.00%> (ø)
rust/src/services/messages/state_machine.rs	0.00% <0.00%> (ø)
rust/src/services/mod.rs	0.00% <ø> (ø)
rust/src/state_machine/mod.rs	45.00% <0.00%> (ø)
rust/src/utils/request.rs	54.54% <54.54%> (ø)
rust/src/state_machine/phases/idle.rs	90.90% <60.00%> (ø)
rust/src/state_machine/phases/error.rs	85.71% <66.66%> (ø)
... and 11 more

---

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[Robert-Steiner]

As already mentioned, really nice improvement!
I only added two questions

[finiteprods]

great - I like that we can remove a lot of code by being less generic! especially in the state machine.
just spotted some undocumented types along the way, nothing major.