docker: add monitoring example

Spinning spdk, telegraf and prometheus containers.
telegraf fetches spdk bdev stats via `bdev_get_iostat`.
prometheus fetches metrics from telegraf.

README also updat

docker: add monitoring example

Spinning spdk, telegraf and prometheus containers.
telegraf fetches spdk bdev stats via `bdev_get_iostat`.
prometheus fetches metrics from telegraf.

README also updated to reflect this example.

Change-Id: I4e7347a1bcd7aca67b4e5b52ebbe662d99f29a3a
Signed-off-by: Boris Glimcher <Boris.Glimcher@emc.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/20332
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>

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