Allow adding a custom string to pip's User-Agent via an environment variable

docs/html/user_guide.rst

@@ -62,6 +62,8 @@ pip can be configured to connect through a proxy server in various ways:
 * using ``proxy`` in a :ref:`config-file`
 * by setting the standard environment-variables ``http_proxy``, ``https_proxy``
   and ``no_proxy``.
+* using the environment variable ``PIP_USER_AGENT_USER_DATA`` to include
+  a JSON-encoded string in the user-agent variable used in pip's requests.
 
 
 .. _`Requirements Files`:

news/5549.feature

@@ -0,0 +1,2 @@
+A custom (JSON-encoded) string can now be added to pip's User-Agent
+using the ``PIP_USER_AGENT_USER_DATA`` environment variable.

src/pip/_internal/download.py

@@ -167,6 +167,10 @@ def user_agent():
     # value to make it easier to know that the check has been run.
     data["ci"] = True if looks_like_ci() else None
 
+    user_data = os.environ.get("PIP_USER_AGENT_USER_DATA")
+    if user_data is not None:
+        data["user_data"] = user_data
+
     return "{data[installer][name]}/{data[installer][version]} {json}".format(
         data=data,
         json=json.dumps(data, separators=(",", ":"), sort_keys=True),

tests/unit/test_download.py

@@ -88,6 +88,11 @@ def test_user_agent__ci(monkeypatch, name, expected_like_ci):
     assert ('"ci":null' in user_agent) == (not expected_like_ci)
 
 
+def test_user_agent_user_data(monkeypatch):
+    monkeypatch.setenv("PIP_USER_AGENT_USER_DATA", "some_string")
+    assert "some_string" in PipSession().headers["User-Agent"]
+
+
 class FakeStream(object):
 
     def __init__(self, contents):