client: allow callers to add information to the user agent (#4690)

This change introduces a new flag to allow callers to add information to
the user agent without replacing it entirely.

This allows people re-packaging or wrapping Certbot to influence its user
agent string. They may which to do this so that stats/metrics related to
their distribution are available to boulder.

This is beneficial for both the Certbot team and the party re-packaging
Certbot as it allows the custom user agent to match the Certbot user
agent as closely as possible, allowing data about use of the re-packaged
version to be collected along side or separately from vanilla certbot.

certbot/client.py

@@ -58,11 +58,12 @@ def determine_user_agent(config):
     # policy, talk to a core Certbot team member before making any
     # changes here.
     if config.user_agent is None:
-        ua = ("CertbotACMEClient/{0} ({1}; {2}) Authenticator/{3} Installer/{4} "
+        ua = ("CertbotACMEClient/{0} ({1}; {2}{8}) Authenticator/{3} Installer/{4} "
               "({5}; flags: {6}) Py/{7}")
         ua = ua.format(certbot.__version__, cli.cli_command, util.get_os_info_ua(),
                        config.authenticator, config.installer, config.verb,
-                       ua_flags(config), platform.python_version())
+                       ua_flags(config), platform.python_version(),
+                       "; " + config.user_agent_comment if config.user_agent_comment else "")
     else:
         ua = config.user_agent
     return ua