plugin XEP-0033: code modernisation, improve delivery, data validation: - Code has been rewritten using Pydantic models and `async` coroutines for data validation and cleaner element parsing/generation. - Delivery has been completely rewritten. It now works even if server doesn't support multicast, and send to local multicast service first. Delivering to local multicast service first is due to bad support of XEP-0033 in server (notably Prosody which has an incomplete implementation), and the current impossibility to detect if a sub-domain service handles fully multicast or only for local domains. This is a workaround to have a good balance between backward compatilibity and use of bandwith, and to make it work with the incoming email gateway implementation (the gateway will only deliver to entities of its own domain). - disco feature checking now uses `async` corountines. `host` implementation still use Deferred return values for compatibility with legacy code. rel 450

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ping: get response time of an XMPP entity
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``ping`` is more or less the equivalent of the command of the same name on most OSes, but
for XMPP. It send a `XEP-0199`_ PING and wait for the answer. When (and if) received, the
time to receive the answer is shown, else the time to receive the error message is shown.
This can be helpful to quickly test the connection with the server or a device.

If you need to get only the response time (without text around), you may use ``-d,
--delay-only``.

.. _XEP-0199: https://xmpp.org/extensions/xep-0199.html

example
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Get reponse time of a server::

  $ li ping example.org