[NO-TICKET] Minor: Clean up a few classes kept in global variables

**What does this PR do?**

This PR gets rid of two references to classes that were being kept as
global variables in the profiling C extension.

* The one in `stack_recorder.c` wasn't actually needed
* The one in `http_transport.c` was only needed for an upcall to Ruby
  code in a rare error handling case, so I've replaced it with an
  actual lookup, so we don't need to keep the reference

**Motivation:**

Having as little global state as possible seems like a good idea, and
we don't need to care about telling or not telling the Ruby GC about
these.

**Additional Notes:**

N/A

**How to test the change?**

The change in `http_transport.c` already has code coverage -- I checked
this by quickly writing the wrong code and validating that indeed
this code was exercised and some of the tests failed as expected.

ext/ddtrace_profiling_native_extension/http_transport.c

@@ -16,7 +16,6 @@ static ID agent_id; // id of :agent in Ruby
 
 static ID log_failure_to_process_tag_id; // id of :log_failure_to_process_tag in Ruby
 
-static VALUE http_transport_class = Qnil;
 static VALUE library_version_string = Qnil;
 
 struct call_exporter_without_gvl_arguments {
@@ -54,7 +53,7 @@ static void interrupt_exporter_call(void *cancel_token);
 static VALUE ddtrace_version(void);
 
 void http_transport_init(VALUE profiling_module) {
-  http_transport_class = rb_define_class_under(profiling_module, "HttpTransport", rb_cObject);
+  VALUE http_transport_class = rb_define_class_under(profiling_module, "HttpTransport", rb_cObject);
 
   rb_define_singleton_method(http_transport_class, "_native_validate_exporter",  _native_validate_exporter, 1);
   rb_define_singleton_method(http_transport_class, "_native_do_export",  _native_do_export, 12);
@@ -180,6 +179,10 @@ static ddog_Vec_Tag convert_tags(VALUE tags_as_array) {
 }
 
 static VALUE log_failure_to_process_tag(VALUE err_details) {
+  VALUE datadog_module = rb_const_get(rb_cObject, rb_intern("Datadog"));
+  VALUE profiling_module = rb_const_get(datadog_module, rb_intern("Profiling"));
+  VALUE http_transport_class = rb_const_get(profiling_module, rb_intern("HttpTransport"));
+
   return rb_funcall(http_transport_class, log_failure_to_process_tag_id, 1, err_details);
 }
 

ext/ddtrace_profiling_native_extension/stack_recorder.c

@@ -129,8 +129,6 @@
 static VALUE ok_symbol = Qnil; // :ok in Ruby
 static VALUE error_symbol = Qnil; // :error in Ruby
 
-static VALUE stack_recorder_class = Qnil;
-
 // Note: Please DO NOT use `VALUE_STRING` anywhere else, instead use `DDOG_CHARSLICE_C`.
 // `VALUE_STRING` is only needed because older versions of gcc (4.9.2, used in our Ruby 2.2 CI test images)
 // tripped when compiling `enabled_value_types` using `-std=gnu99` due to the extra cast that is included in
@@ -217,7 +215,7 @@ static VALUE _native_record_endpoint(DDTRACE_UNUSED VALUE _self, VALUE recorder_
 static void reset_profile(ddog_prof_Profile *profile, ddog_Timespec *start_time /* Can be null */);
 
 void stack_recorder_init(VALUE profiling_module) {
-  stack_recorder_class = rb_define_class_under(profiling_module, "StackRecorder", rb_cObject);
+  VALUE stack_recorder_class = rb_define_class_under(profiling_module, "StackRecorder", rb_cObject);
   // Hosts methods used for testing the native code using RSpec
   VALUE testing_module = rb_define_module_under(stack_recorder_class, "Testing");