tcp_metrics: add json support

tcp_metrics: add json support

Add JSON support to the ip tcp_metrics output.

$ ip -j -p tcp_metrics show
[ {
        "dst": "192.18.1.11",
        "age": 23617.8,
        "ssthresh": 7,
        "cwnd": 3,
        "rtt": 0.039176,
        "rttvar": 0.039176,
        "source": "192.18.1.2"
    }
...

The JSON output does scale values differently since there is no good
way to indicate units. The rtt values are displayed in seconds in
JSON and microseconds in the original (non JSON) mode. In the example
above the output in without the -j flag, the output would be
 ... rtt 39176us rttvar 39176us

I did this since all the other values in the JSON record are also in
floating point seconds.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Ahern <dsahern@gmail.com>