feat!: add a dep on Abseil

[devjgm]

Fixes: #2309

Adds a dep on Abseil. Uses absl::make_unique in one test in Spanner just to show that the Abseil dep is working in all the builds.

This change is 

[codecov]

Codecov Report

Merging #4087 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4087   +/-   ##
=======================================
  Coverage   93.34%   93.34%           
=======================================
  Files         834      835    +1     
  Lines       64928    64884   -44     
=======================================
- Hits        60607    60566   -41     
+ Misses       4321     4318    -3     

Impacted Files	Coverage Δ
google/cloud/spanner/results_test.cc	93.75% <100.00%> (ø)
google/cloud/storage/oauth2/credentials.h	33.33% <0.00%> (-33.34%)	⬇️
google/cloud/storage/internal/curl_handle.h	68.18% <0.00%> (-31.82%)	⬇️
...oogle/cloud/storage/internal/curl_handle_factory.h	50.00% <0.00%> (-30.00%)	⬇️
google/cloud/testing_util/assert_ok.cc	80.00% <0.00%> (-20.00%)	⬇️
google/cloud/spanner/internal/log_wrapper.h	71.42% <0.00%> (-13.19%)	⬇️
google/cloud/internal/async_retry_unary_rpc.h	75.51% <0.00%> (-6.13%)	⬇️
...storage/benchmarks/storage_throughput_benchmark.cc	83.63% <0.00%> (-5.66%)	⬇️
google/cloud/storage/bucket_metadata.h	99.41% <0.00%> (-0.02%)	⬇️
google/cloud/optional.h	100.00% <0.00%> (ø)
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1f9ab3b...3f50b82. Read the comment docs.

[coryan]

Reviewed 16 of 17 files at r1, 2 of 2 files at r2.
Reviewable status: 18 of 19 files reviewed, all discussions resolved

---

.clang-format, line 17 at r2 (raw file):

- Regex: '^\"google/cloud/'  # project includes should sort first
  Priority: 500
- Regex: '^\"'

FYI, by including abseil using #include "abseil/...." we are missing out on this:

https://github.com/googleapis/google-cloud-cpp-pubsub/blob/6db6162cb6bbe9bedb04933ef8ec073386dbda7f/cmake/EnableWerror.cmake#L29-L39

That allows us to increase the warning level in our code, even if our dependencies are not ready.

---

super/CMakeLists.txt, line 55 at r2 (raw file):

set(GOOGLE_CLOUD_CPP_DEPENDENCIES_LIST
    abseil-cpp-project

FYI, when we upgrade grpc-project we need to change its deps to add abseil-cpp-project to them.

[devjgm]

Reviewed 16 of 17 files at r1, 2 of 2 files at r2.
Reviewable status: 18 of 19 files reviewed, all discussions resolved

.clang-format, line 17 at r2 (raw file):

- Regex: '^\"google/cloud/'  # project includes should sort first
  Priority: 500
- Regex: '^\"'

FYI, by including abseil using #include "abseil/...." we are missing out on this:

https://github.com/googleapis/google-cloud-cpp-pubsub/blob/6db6162cb6bbe9bedb04933ef8ec073386dbda7f/cmake/EnableWerror.cmake#L29-L39

That allows us to increase the warning level in our code, even if our dependencies are not ready.

Including Abseil headers with angle brackets breaks the bazel builds. I know we talked about making this work with them, but it was either forgotten or it was decided not to do it.

If our build breaks because of warnings in Abseil headers, we could file issues and escalate to them. They'll likely either need to fix the breakages quickly or perhaps enable angle-bracket includes.

Or we could avoid the dep until they enable angle-bracket includes of their headers (Note: we currently don't support angle-bracket includes of our headers in bazel builds either).

WDYT?

super/CMakeLists.txt, line 55 at r2 (raw file):

set(GOOGLE_CLOUD_CPP_DEPENDENCIES_LIST
    abseil-cpp-project

FYI, when we upgrade grpc-project we need to change its deps to add abseil-cpp-project to them.

Gotcha. Thanks.

[coryan]

Reviewed 1 of 17 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

---

.clang-format, line 17 at r2 (raw file):

Previously, devjgm (Greg Miller) wrote…

Including Abseil headers with angle brackets breaks the bazel builds. I know we talked about making this work with them, but it was either forgotten or it was decided not to do it.

If our build breaks because of warnings in Abseil headers, we could file issues and escalate to them. They'll likely either need to fix the breakages quickly or perhaps enable angle-bracket includes.

Or we could avoid the dep until they enable angle-bracket includes of their headers (Note: we currently don't support angle-bracket includes of our headers in bazel builds either).

WDYT?

SGTM. FYI, I am preparing a PR to start using -Werror on Windows (we were not, it finds some useful stuff).