Doc additions for Simple and Batch processors

[cijothomas]

Fixes #2189

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.8%. Comparing base (90b0dd4) to head (643b1f6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2529   +/-   ##
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  Coverage   77.8%   77.8%           
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  Files        123     123           
  Lines      23059   23059           
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  Hits       17940   17940           
  Misses      5119    5119           

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[lalitb]

Nicely documented.

[lalitb]

should we mention this is due to limitation from the reqwest crate, and not a restriction imposed by us?

[cijothomas]

lets improve this based on any feedback we get. I am neutral about this currently, since we talk about supported features flags in general, without talking about why something is not supported.

[ratijas]

This is actually so very well hidden, considering that scattered examples mostly use builder method with_simple_exporter which doesn't really expose the issue with SimpleSpanProcessor; so you have to find this thing among the docs, or read through the source code

[ratijas]

Also, what exactly does this mean?

The following feature flags offer additional configurations on http:

As far as my reverse-engineering skills go, these feature flags offer no configuration. In fact, it's just a big if { #[cfg(...)] #[cfg(...)] #[cfg(...)] } in another crate which will kinda panic if you got the cfg part wrong. Which you will get wrong on the first try, because you forgot to remove default-features before adding the right flavor of client. AFAICT nothing mentions those client features are supposed to be mutually exclusive either. You will get it wrong on the second try too, because you just broke the build, so now you have to add all the default features back one by one — all but the "reqwest-blocking-client" obviously. Phew…

…and after all that setup, it just hang up at
DEBUG reqwest::connect > starting new connection: http://localhost:4318/
not even able to be interrupted by Ctrl+C.

Something isn't so Simple about this Simple processor, huh