fix: improve pg_stat_statements individual metric panels

[keithf4]

Description

Lifetime stats should be using the instant max value. Mean causes Grafana to average out all values which makes no sense, even if you do ask for a time range

Made new time range panels for the same value panels to reflect the requested time range values. It takes the difference between the min and max values for that range. Since these are always incrementing values, the low time range should always be the min and high time range the max. Grafana still does weird "smoothing" on these values, even if you chose the max possible time range, so they never match the lifetime values. Hence marking them Est.

Doing a time range value for the mean doesn't make sense with the way the metric is gathered right now, so left that one out.

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• Bugfix
• Enhancement
• Breaking Change
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closes #449

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Testing

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Code testing

Have you tested your changes against:

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If your code touches sql_exporter, have you:

• Tested against all versions of PostgreSQL affected
• Ensure that exporter runs with no scrape errors
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Checklist:

• I have made corresponding changes to:
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  • the release notes
  • the upgrade doc