Fix profiler wrongly detecting that google-protobuf is installed

[ivoanjo]

What does this PR do?:

The google-protobuf gem is a dependency needed by the profiler (that we plan on removing when we fully move to libdatadog), but that is not a dependency of the ddtrace gem.

Because it's not a dependency of ddtrace, we need to gracefully handle the case where this gem is missing. Thus, before requiring any files from this gem, the profiler uses rubygems to check if the gem is available.

But, in #1506, a customer asked us to allow skipping this API call to rubygems, since it didn't work on their setup. Instead, we added an extra check -- if the ::Google::Protobuf module is available, then we assume protobuf is installed.

Very very unfortunately, there's actually another gem, called protobuf (or protobuf-cucumber, its fork) that is not compatible with ddtrace BUT STILL defines ::Google::Protobuf.

This is horrifying -- having two different gems that define different versions of a module with the same name.

But in this very bizarre situation, the google-protobuf detection code got confused, and one internal customer reported that he saw the following error on their logs:

[DDTRACE] Error while loading google-protobuf gem. Cause: 'LoadError cannot load such file -- google/protobuf' Location: 'lib/datadog/profiling.rb:106:in `require''. This can happen when google-protobuf is missing its native components. [...]

To fix this, I've tweaked the google-protobuf detection to also check for the ::Protobuf module. This top-level module is never defined by google-protobuf, and we can use it to distinguish between the two gems.

Motivation:

Avoid misleading error message when loading ddtrace with protobuf/protobuf-cucumber installed.

Additional Notes:

I'm looking forward to dropping this whole google-protobuf code once the new profiler becomes the default for all customers.

How to test the change?:

The following trivial Ruby app is able to trigger the issue.

• gems.rb:

source 'http://rubygems.org'
gem 'protobuf-cucumber'
gem 'ddtrace'

• example.rb:

require 'protobuf'
require 'ddtrace'
puts "Works!"

[codecov-commenter]

Codecov Report

Merging #2595 (47340ba) into master (2f31cd8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2595      +/-   ##
==========================================
- Coverage   98.04%   98.04%   -0.01%     
==========================================
  Files        1143     1143              
  Lines       62015    62021       +6     
  Branches     2789     2789              
==========================================
+ Hits        60804    60809       +5     
- Misses       1211     1212       +1     

Impacted Files	Coverage Δ
lib/datadog/profiling.rb	100.00% <100.00%> (ø)
spec/datadog/profiling_spec.rb	100.00% <100.00%> (ø)
lib/datadog/core/workers/polling.rb	96.55% <0.00%> (-3.45%)	⬇️
lib/datadog/tracing/distributed/propagation.rb	91.11% <0.00%> (-2.23%)	⬇️
...atadog/tracing/contrib/grpc/support/grpc_helper.rb	98.24% <0.00%> (-1.76%)	⬇️
...ng/contrib/active_support/cache/instrumentation.rb	86.20% <0.00%> (-0.10%)	⬇️
...dog/profiling/collectors/cpu_and_wall_time_spec.rb	97.88% <0.00%> (+0.42%)	⬆️

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