chore(build): use includes argument in BUILD.bazel

[dmehala]

Description

Envoy was failing to build becase it couldn't locate the necessary headers in the src directory, even with the -Isrc/datadog flag. After investigating, I found that dd-trace-cpp srouce code is actually located at external/com_github_datadog_dd_trace_cpp/src/datadog. This means the correct include path should be -Iexternal/com_github_datadog_dd_trace_cpp/src/datadog. For some reson Bazel isn't able to handle this path expansion automatically.

After researching, it seems the recommended solution is to use the includes argument in cc_library. However, the Bazel documentation includes this warning:

This should only be used for third-party libraries that do not conform to the Google style of writing #include statements. Unlike COPTS, these flags are added for this rule and every rule that depends on it. (Note: not the rules it depends upon!) Be very careful, since this may have far-reaching effects.

I'm not entirely sure about the implications here, so I went ahead with this approach despite the warning. If someone with a bigger brain, IQ and experience can suggest a better way™️, I'm happy to adjust accordingly!

Motivation

Preparing the new release 🎉