Adds support for custom metadata for aws-elixir.

This adds support for custom metadata for elixir (see #92). It also
adds code that was removed in #103 which appears to have inadvertently
removed erlang support, as well as functions necessary for elixir
support (e.g., `collect_request_headers_parameters`).

lib/aws_codegen/rest_service.ex

@@ -256,6 +256,9 @@ defmodule AWS.CodeGen.RestService do
       function_name = AWS.CodeGen.Name.to_snake_case(operation)
       request_header_parameters = collect_request_header_parameters(language, api_spec, operation)
 
+      request_headers_parameters =
+        collect_request_headers_parameters(language, api_spec, operation)
+
       is_required = fn param -> param.required end
       required_query_parameters = Enum.filter(query_parameters, is_required)
       required_request_header_parameters = Enum.filter(request_header_parameters, is_required)
@@ -266,7 +269,8 @@ defmodule AWS.CodeGen.RestService do
           "GET" ->
             case language do
               :elixir ->
-                2 + length(request_header_parameters) + length(query_parameters)
+                2 + length(request_header_parameters) + length(request_headers_parameters) +
+                  length(query_parameters)
 
               :erlang ->
                 4 + length(required_request_header_parameters) + length(required_query_parameters)
@@ -295,6 +299,7 @@ defmodule AWS.CodeGen.RestService do
         query_parameters: query_parameters,
         required_query_parameters: required_query_parameters,
         request_header_parameters: request_header_parameters,
+        request_headers_parameters: request_headers_parameters,
         required_request_header_parameters: required_request_header_parameters,
         response_header_parameters:
           collect_response_header_parameters(language, api_spec, operation),
@@ -338,6 +343,10 @@ defmodule AWS.CodeGen.RestService do
     collect_parameters(language, api_spec, operation, "input", "smithy.api#httpHeader")
   end
 
+  defp collect_request_headers_parameters(language, api_spec, operation) do
+    collect_parameters(language, api_spec, operation, "input", "smithy.api#httpPrefixHeaders")
+  end
+
   defp collect_response_header_parameters(language, api_spec, operation) do
     collect_parameters(language, api_spec, operation, "output", "smithy.api#httpHeader")
   end

priv/rest.ex.eex

@@ -124,7 +124,13 @@ def <%= action.function_name %>(%Client{} = client<%= AWS.CodeGen.RestService.fu
         {"<%= parameter.name %>", "<%= parameter.location_name %>"},<% end %>
       ]
       |> Request.build_params(input)<% else %>
-    headers = []<% end %><%= if length(action.query_parameters) > 0 do %>
+    headers = []<% end %><%= if length(action.request_headers_parameters) > 0 do %>
+    {custom_headers, input} =
+      [<%= for parameter <- action.request_headers_parameters do %>
+        {"<%= parameter.name %>", "<%= parameter.location_name %>"},<% end %>
+      ]
+      |> Request.build_params(input)<% else %>
+    custom_headers = []<% end %><%= if length(action.query_parameters) > 0 do %>
     {query_params, input} =
       [<%= for parameter <- action.query_parameters do %>
         {"<%= parameter.name %>", "<%= parameter.location_name %>"},<% end %>
@@ -168,6 +174,6 @@ def <%= action.function_name %>(%Client{} = client<%= AWS.CodeGen.RestService.fu
         metadata()
       <% end %>
 
-    Request.request_rest(client, meta, <%= AWS.CodeGen.RestService.Action.method(action) %>, url_path, query_params, headers, input, options, <%= inspect(action.success_status_code) %>)<% end %>
+    Request.request_rest(client, meta, <%= AWS.CodeGen.RestService.Action.method(action) %>, url_path, query_params, custom_headers ++ headers, input, options, <%= inspect(action.success_status_code) %>)<% end %>
   end<% end %>
 end