[runner subcmd] init run subcommand

[fuweid]

• Add --clients Total number of HTTP clients flag option

Currently, that runner uses --conns number as client number. By default, client-go uses HTTP2 protocol. One connection can serve multiple requests in parallel. Runner should reuse that conns to handle requests if --clients number is larger than --conns.

• Add ObserveReceivedBytes metric method for ResponsesStats

For GET/LIST requests, the client-go takes long time to receive huge responses. The TotalReceivedBytes metric can help us to understand total traffic bytes and average RX rate during testing.

• [runner subcmd] support -v flag for run #32

• Failure in ResponseStats should be partitioned by HTTP verb and HTTP code

NOTE: #15 is good enough. Ideally, we should not have a ton of errors.

• Add --result=path flag to export result into specific file instead of stdout

Tasks

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1. cmd: add --clients and --content-type flags to runner command #23
   +0
2. *: add TotalReceivedBytes metric and --result flag to runner command #21
   +0
3. feat: observe error instead of failure count #15
   +0
4. [runner subcmd] support -v flag for run #32
   kind/feature priority/P3 +2
   
5. [runner subcmd] add --raw-data flag to export raw data instead of aggregated one. #39
   kind/feature priority/P2 +2