interval: only sleep off remainder

[yaauie]

Attempt to adhere to the interval, regardless of how long execution takes.
After executing, sleep off only the remainder of the interval (if any), and
log a helpful warning when execution takes longer than the interval.

Release notes

Fixes issues related to fetching SNMP data at a periodic interval. The plugin now only sleeps off the remainder of the interval between runs, and logs a helpful warning when execution takes longer than the configured interval.

What does this PR do?

Introduces and tests a new StoppableIntervalRunner, which is used to run the crawler at a periodic interval. The implementation respects the stop-status of the plugin, and uses the plugin's logger to warn when execution takes longer than the configured interval.

Why is it important/What is the impact to the user?

Ensures that the interval option supports actually collecting SNMP data at that periodic interval. When this plugin is configured with a 5-minute interval (interval => 300), and a number of hosts that take anywhere from 3-seconds to 1-minute to fetch data from, this ensures that the start of each crawl is correctly 5-minutes apart.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Related issues

Resolves: #61

Logs

When the interval cannot be satisfied, a helpful log message is emitted:

[2021-09-14T17:54:05,585][WARN ][logstash.input.snmp    ][my-snmp-pipeline] polling hosts took longer than the configured interval {:interval_seconds=>30, :duration_seconds=>37.019}

[andsel]

Nice PR

LGTM