Fix profiler wrongly detecting that google-protobuf is installed

**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:

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

* example.rb:

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

lib/datadog/profiling.rb

@@ -65,7 +65,7 @@ def self.start_if_enabled
       # NOTE: On environments where protobuf is already loaded, we skip the check. This allows us to support environments
       # where no Gem.loaded_version is NOT available but customers are able to load protobuf; see for instance
       # https://github.com/teamcapybara/capybara/commit/caf3bcd7664f4f2691d0ca9ef3be9a2a954fecfb
-      if !defined?(::Google::Protobuf) && Gem.loaded_specs['google-protobuf'].nil?
+      if !protobuf_already_loaded? && Gem.loaded_specs['google-protobuf'].nil?
         "Missing google-protobuf dependency; please add `gem 'google-protobuf', '~> 3.0'` to your Gemfile or gems.rb file"
       end
     end
@@ -74,12 +74,16 @@ def self.start_if_enabled
       # See above for why we skip the check when protobuf is already loaded; note that when protobuf was already loaded
       # we skip the version check to avoid the call to Gem.loaded_specs. Unfortunately, protobuf does not seem to
       # expose the gem version constant elsewhere, so in that setup we are not able to check the version.
-      if !defined?(::Google::Protobuf) && Gem.loaded_specs['google-protobuf'].version < GOOGLE_PROTOBUF_MINIMUM_VERSION
+      if !protobuf_already_loaded? && Gem.loaded_specs['google-protobuf'].version < GOOGLE_PROTOBUF_MINIMUM_VERSION
         'Your google-protobuf is too old; ensure that you have google-protobuf >= 3.0 by ' \
         "adding `gem 'google-protobuf', '~> 3.0'` to your Gemfile or gems.rb file"
       end
     end
 
+    private_class_method def self.protobuf_already_loaded?
+      defined?(::Google::Protobuf) && !defined?(::Protobuf)
+    end
+
     private_class_method def self.protobuf_failed_to_load?
       unless protobuf_loaded_successfully?
         'There was an error loading the google-protobuf library; see previous warning message for details'

spec/datadog/profiling_spec.rb

@@ -133,6 +133,16 @@
           end
 
           it { is_expected.to be nil }
+
+          context "but it's the protobuf/cucumber-protobuf gem instead of google-protobuf" do
+            include_context 'loaded gems', :'google-protobuf' => nil
+
+            before do
+              stub_const('::Protobuf', Module.new)
+            end
+
+            it { is_expected.to include 'Missing google-protobuf' }
+          end
         end
       end
     end