[BUILD] Restore Bazel flag removed from public API

[dbolduc]

A follow up to #2679.

This bool_flag was a part of the public API. If we remove it, anyone using it in downstream code have broken build commands.

So instead of removing it, I think we should mark it as deprecated.

$ bazelist build --//api:with_abseil //api:api

WARNING: opentelemetry-cpp/api/BUILD:8:10: target '//api:with_abseil' is deprecated: The value of this flag is ignored. Bazel builds always use Abseil's pre-adopted `std::` types. You should remove this flag from your build command.
INFO: Analyzed target //api:api (0 packages loaded, 545 targets configured).
INFO: Found 1 target...
Target //api:api up-to-date (nothing to build)
INFO: Elapsed time: 0.574s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

---

On removing the now useless flag...

I do not know what OpenTelemetry's breaking change policy is. But consider something like:

• removing the flag in the next major version bump (v2.x). That is Google's policy (don't laugh).
• OR giving the flag a few releases in the deprecated state, so downstream projects have a wider window to remove it.

In my own repo, I would typically open a new issue and tag the code so we remember to clean it up later.

TODO(#XXXX) - Remove this flag when ...
bool_flag( ... )

I am happy to do that here. You will have to tell me when you want to remove it. (If I have convinced y'all to take this PR).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.67%. Comparing base (497eaf4) to head (cbceed1).
Report is 82 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2702      +/-   ##
==========================================
+ Coverage   87.12%   87.67%   +0.55%     
==========================================
  Files         200      190      -10     
  Lines        6109     5853     -256     
==========================================
- Hits         5322     5131     -191     
+ Misses        787      722      -65     

see 106 files with indirect coverage changes

[marcalff]

LGTM, thanks for the fix.

[marcalff]

@keith Please take a look and comment.

I assume keeping a unused flag is ok.

[owent]

LGTM, thanks.

[keith]

Keeping and marking deprecated is definitely on. It might be a bit misleading if folks expect it to do something but I definitely understand the issue.