message: style guide: Describe Envoy naming convention for constants
  Calling out Envoy's constant naming convention and its difference from the Google style guide
diff: advice in the [C++ FAQ on the static initialization
  fiasco](https://isocpp.org/wiki/faq/ctors#static-init-order-on-first-use) for
  how to best handle this.
  +* The Google C++ style guide points out that [constant vars should be named `kConstantVar`](https://google.github.io/styleguide/cppguide.html#Constant_Names).
  + In the Envoy codebase we use `ConstantVar` or `CONSTANT_VAR`. If you pick `CONSTANT_VAR`,
  + please be certain the name is globally significant to avoid potential conflicts with #defines,
  + which are not namespace-scoped, and may appear in externally controlled header files.
  * API-level comments should follow normal Doxygen conventions. Use `@param` to describe
  parameters and `@return <return-type>` for return values. Internal comments for
  methods and member variables may be regular C++ `//` comments or Doxygen at
  