Fix time elapsed (#1277)

* Show the actual time elapsed; add docs

* `requests.Response._headers_parsed_at` → `requests.Response._httpie_headers_parsed_at`

* Add `ELAPSED_TIME_LABEL` constant

* Tweak docs

* Tweak docs

* Allow multiple blank lines in Markdown files

* Add rudimentary tests for --meta with different --style’s

* Cleanup tests

* Cleanup tests

* Cleanup tests

httpie/client.py

@@ -3,6 +3,7 @@ import http.client
 import json
 import sys
 from contextlib import contextmanager
+from time import monotonic
 from typing import Any, Dict, Callable, Iterable
 from urllib.parse import urlparse, urlunparse
 
@@ -108,7 +109,7 @@ def collect_messages(
                     **send_kwargs_merged,
                     **send_kwargs,
                 )
-
+            response._httpie_headers_parsed_at = monotonic()
             expired_cookies += get_expired_cookies(
                 response.headers.get('Set-Cookie', '')
             )