Add RTT to Sum when using JSON

[hajoha]

Context

• Version of iperf3:
  all

• Operating system (and distribution, if any):
  all

Enhancement Request

• Current behavior
  Currently the RTT stats are not shown in the Sum Object

	"streams":	[{
			"socket":	6,
			"start":	1.001069,
			"end":	2.000895,
			"seconds":	0.99982601404190063,
			"bytes":	11927552,
			"bits_per_second":	95437020.7014849,
			"retransmits":	15,
			"snd_cwnd":	139152,
			"snd_wnd":	3142272,
			"rtt":	13505,
			"rttvar":	994,
			"pmtu":	1422,
			"omitted":	false,
			"sender":	true
		}],
	"sum":	{
		"start":	1.001069,
		"end":	2.000895,
		"seconds":	0.99982601404190063,
		"bytes":	11927552,
		"bits_per_second":	95437020.7014849,
		"retransmits":	15,
		"omitted":	false,
		"sender":	true
	}

• Desired behavior
  Display the RTT stats for each interval also in the sum object

	"streams":	[{
			"socket":	6,
			"start":	1.001069,
			"end":	2.000895,
			"seconds":	0.99982601404190063,
			"bytes":	11927552,
			"bits_per_second":	95437020.7014849,
			"retransmits":	15,
			"snd_cwnd":	139152,
			"snd_wnd":	3142272,
			"rtt":	13505,
			"rttvar":	994,
			"pmtu":	1422,
			"omitted":	false,
			"sender":	true
		}],
	"sum":	{
		"start":	1.001069,
		"end":	2.000895,
		"seconds":	0.99982601404190063,
		"bytes":	11927552,
		"bits_per_second":	95437020.7014849,
		"retransmits":	15,
		"omitted":	false,
		"rtt":	13505,
		"rttvar":	994,
		"sender":	true
	}

• Implementation notes
  I'm curious why this hasn't been implemented yet. Is it because the mean RTT for the streams is considered useless, or has it simply been overlooked? I can handle the implementation, but I wanted to check before getting started.
  For multiple streams i would choose mean.