MySQL plugin should not collect global variables at each telegraf data collection interval

[lgosselin]

When you set interval_slow (to a positive value), MySQL plugin starts gathering the global variables.

What is expected (in accordance with the README.md): The gathering of variables occurs at a slower pace than the regular telegraf data collection interval. And the pace would be the value interval_slow (+ an eventual jitter of 1 telegraf interval).

What is actually happening: The plugin collects the global variable at every single data collection interval.

The fix is not to call the gathering when it was done less than interval_slow duration ago.

Note:

• I've signed the CLA
• I did not change the README.md. This pull request is to make the code behave as advertised in the README.md.
• I don't know how to produce a unit test that validates the timing. I've checked by looking at the timestamps in influxdb.

[danielnelson]

Thank you!