add root path command options

[martian4202]

add root-path command option in case asynqmon running behind a proxy

[harry0703]

h := asynqmon.New(asynqmon.Options{
		RedisConnOpt:      redisConnOpt,
		PayloadFormatter:  asynqmon.PayloadFormatterFunc(payloadFormatterFunc(cfg)),
		ResultFormatter:   asynqmon.ResultFormatterFunc(resultFormatterFunc(cfg)),
		PrometheusAddress: cfg.PrometheusServerAddr,
		ReadOnly:          cfg.ReadOnly,
                 // ***** set the RootPath *****
		RootPath:          cfg.RootPath,
	})

[evgenyKharitonov]

Hey @hibiken can you please merge this so we can open another PR to add ENV var support for this?

Thanks!

[taltcher]

Hey @hibiken can you please merge this so we can open another PR to add ENV var support for this?

Thanks!