Change metric type for system_uptime_format in the system input plugin

[ildarsv]

Bug report

Since the uptime_format field was added into the system input plugin as Counter, I have tag uptime_format, not field as expected in my prometheus output. The reason is the following: string value can not be Counter. By the way, prometheus doesn't support string values, in that case system_uptime_format have to be skipped.

Relevant telegraf.conf:

[[inputs.system]]

[[outputs.prometheus_client]]
listen = "0.0.0.0:9126"

System info:

telegraf 1.5 rc1

Steps to reproduce:

1. Add system input and prometheus output.
2. Check system_uptime metric.

Expected behavior:

system_uptime{host="host_name"} 186665

Actual behavior:

system_uptime{host="cfg01",uptime_format="2 days, 3:51"} 186665