Add sendgrid headers

application_mailer.rb

class ApplicationMailer < ActionMailer::Base
  default from: "Example.com <support@example.com>"

  def process(name, *args)
    set_headers_for_sendgrid(name, args)
    super
  end

  private
  
  def set_headers_for_sendgrid(mailer_action, mailer_action_arg_values)
    mailer = self.class.name

    # parameters gives us e.g. [ [:req, :arg1 ], … ] with types and names.
    mailer_action_arg_names = method(mailer_action).parameters.map(&:last)

    # Turn e.g. [ "k1", "k2" ] and ["v1", "v2" ] into { "k1" => "v1", "k2" => "v2" }
    mailer_action_args = mailer_action_arg_names.zip(mailer_action_arg_values).to_h

    headers["X-SMTPAPI"] = {
      category: [ mailer, "#{mailer}##{mailer_action}" ],
      unique_args: {
        environment: Rails.env,
        arguments: mailer_action_args.to_json,
      }
    }.to_json
  end
end