NH-90184: add feature - add_tracer on function

[xuan-cao-swi]

Description

Add the feature that can instrument functions.

Usage
(Both examples should produce an additional span for the new_method function):

For Ruby script:

class MyClass
  include SolarWindsAPM::API::Tracer

  def new_method(param1, param2)
    param1 + param2
  end

  add_tracer :new_method, 'custom_name', { attributes: { 'foo' => 'bar' }, kind: :consumer }
end

my_class = MyClass.new
my_class.new_method(1, 2)

For Rails (e.g., controller):

class PagesController < ApplicationController
  include SolarWindsAPM::API::Tracer

  def new_method(param1, param2)
    param1 + param2
  end

  add_tracer :new_method, 'custom_name', { attributes: { 'foo' => 'bar' }, kind: :consumer }

  # e.g., curl http://0.0.0.0:8002/about
  def about
    new_method(1, 2)
  end
end

Test (if applicable)

Added unit test
Ad-hoc test on rails controller

[jerrytfleung]

LGTM