PCT-565 Protection from invalid caputure

Added a control of time took to collect the metrics.
If it is > 10% of ticker interval, metrics are discarded.
The calculated time include all metrics collected in the run function
because ie if SHOW STATUS takes 3 seconds, all following metrics like
SELECT NAME, ... FROM INFORMATION_SCHEMA.INNODB_METRICS will be
timeshifted, so I think we need to ignore them to keep them synchronized.
Let's change this block to:
```go
   lastError =  fmt.Sprintf("Skipping interval because it took too long to collect: %.1fs >= %ds", d, m.config.Collect)
   m.logger.Warning(lastError)
   continue
```
`lastError` is special; see how it's used in the code. And we must continue else we'll keep doing post-collection stuff (and there's nothing to do if there's no valid data).