--json flag not producing json output in server mode

[jeanielamwh]

Hello, I am using v1.3.1 and noticed that despite passing the --json flag, the output is not in json when server mode is used.

Below is the command used. The output is not in json.

$ warp -mem -cpu -pprofdir xxx mixed --no-color --host xxx --tls --json --concurrent 4 --duration 10s --warp-client xxx --access-key xxx --secret-key xxx --bucket warp-000 --obj.size 4KiB --benchdata xxx
[info] CPU profiling enabled, file xxx
[info] Memory profiling (heap) enabled at rate 4096, file xxx

Report: DELETE. Concurrency: 4. Ran: 11s
 * Average: 140.28 obj/s
 * Reqs: Avg: 2.9ms, 50%: 2.8ms, 90%: 3.3ms, 99%: 6.4ms, Fastest: 2.3ms, Slowest: 11.4ms, StdDev: 0.6ms

Throughput, split into 7 x 1s:
 * Fastest: 228.92 obj/s
 * 50% Median: 219.66 obj/s
 * Slowest: 216.40 obj/s

[...truncated the rest]

If I remove --warp-client xxx then the output is in json.

I think the fix in #409 needs to be applied to https://github.com/minio/warp/blob/v1.3.1/cli/benchserver.go#L318-L322 too.