feat: save the raw response for debugging purpose. (#37)

* feat: save the raw response for debugging purpose.

* refactor: use `raw_response_cb` for exposing raw response

* docs: documentation about `raw_response_cb`.

* fix: use `self.params.raw_response_cb`.

README.md

@@ -406,3 +406,23 @@ cmp.setup({
   },
 })
 ```
+
+## Debugging Information
+
+To retrieve the raw response from the backend, you can set the following option
+in `provider_options`: 
+```lua
+provider_options = {
+  raw_response_cb = function(response)
+    -- the `response` parameter contains the raw response (JSON-like) object.
+
+    vim.notify(vim.inspect(response)) -- show the response as a lua table
+
+    vim.g.ai_raw_response = response -- store the raw response in a global
+                                     -- variable so that you can use it
+                                     -- somewhere else (like statusline).
+  end,
+}
+```
+This provides useful information like context lengths (# of tokens) and 
+generation speeds (tokens per seconds), depending on your backend. 

lua/cmp_ai/requests.lua

@@ -57,6 +57,9 @@ function Service:Get(url, headers, data, cb)
 
         local result = table.concat(response:result(), '\n')
         local json = self:json_decode(result)
+        if type(self.params.raw_response_cb) == 'function' then
+          self.params.raw_response_cb(json)
+        end
         if json == nil then
           cb({ { error = 'No Response.' } })
         else