Submits PING frame.  You don't have to send PING back when you
received PING frame.  The library automatically submits PING frame
in this case.

The *flags* is bitwise OR of 0 or more of the following value.

* :enum:`nghttp2_flag.NGHTTP2_FLAG_ACK`

Unless `nghttp2_option_set_no_auto_ping_ack()` is used, the *flags*
should be :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.

If the *opaque_data* is non ``NULL``, then it should point to the 8
bytes array of memory to specify opaque data to send with PING
frame.  If the *opaque_data* is ``NULL``, zero-cleared 8 bytes will
be sent as opaque data.

This function returns 0 if it succeeds, or one of the following
negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.