1. made Runner sleep for (PollInterval minus processing time)

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1. Logging Processing time every cycle
2. Add PollInterval to Config file and using it instead of interval hardcoded value.

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sidekicks @lars2893 @wquraishi

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1. This commit primarily addresses an issue with the current version where the metrics reporting cycle period can vary, depending on the time it takes to process the data and report the metrics. As a result, plugins that take a long time to process can see missing data points in the middle of a series.

The first idea was to run the poll cycle in its own timer thread. Though more elegant, that approach requires non-trivial changes, especially to the way exceptions are handled and bubbled up. The simpler approach proposed in this PR leaves most of the code in tact, calculates the time it took to process metrics and subtracts it from the poll cycle interval.

1. Made the poll cycle interval a config variable (in newrelic.json) to enable plug-in users to customize the poll cycle interval.
2. Logging the processing time for each cycle and reporting a warning if the time exceeds the poll cycle interval.

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sidekick @lars2893

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