chore: only enable histograms for Redis 7+

[grafanalf]

relates-to: #784

[coveralls]

Pull Request Test Coverage Report for Build 219

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 91.785%

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Files with Coverage Reduction	New Missed Lines	%
exporter/latency.go	5	91.3%

Totals
Change from base Build 218:	-0.2%
Covered Lines:	1944
Relevant Lines:	2118

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💛 - Coveralls

[oliver006]

Thanks for the PR!

I think I mentioned in #784 that I prefer to not make the exporter version-aware, esp if it's not needed to enable certain functionality.
The log-line should show up only once - is it that big of an issue?