Experimental fixes

[albertored]

A couple of improvements on the experimental applications.

Different contributions are split in different commits so if you prefer to accept only some of them I can discard the unwanted ones:

• Improvement on typespecs
• Easier way for creating observable instrumenters without callback
• Add instrument_unit to view criteria (see spec)
• Validate instrument name (see spec)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (23c49ec) 72.26% compared to head (ca16631) 72.26%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   72.26%   72.26%           
=======================================
  Files          60       60           
  Lines        1904     1904           
=======================================
  Hits         1376     1376           
  Misses        528      528           

Flag	Coverage Δ
api	68.06% <ø> (ø)
elixir	18.31% <ø> (ø)
erlang	73.58% <ø> (ø)
exporter	66.80% <ø> (ø)
sdk	78.13% <ø> (ø)
zipkin	54.16% <ø> (ø)

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[albertored]

@tsloughter review suggestion applied and rebased on main, tests pass locally

[albertored]

I think the failing test is a little bit flaky, it fails (not always) also on different PR (see for instance #609)

[tsloughter]

Yup, I'm rerunning it.Should be fine.

[tsloughter]

Hey, can you send another PR with an update to the changelog? Sorry, I always forget to check this until I hit the merge button :)

[albertored]

Also changes to experimental apps go into the main changelog?

[tsloughter]

Yea, just under their names, so like Experimental API is the heading.