index: add SampleValues to Statistics interface

[dimitarvdimitrov]

Add SampleValues method to the Statistics interface to allow selectivity estimation based on actual label values.

This is groundwork for improving regex selectivity estimation in query planning.

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Note

Adds SampleValues(ctx, name) []string to Statistics for retrieving representative label value samples.

• API (index):
  • Extend Statistics with SampleValues(ctx, name) []string to fetch representative samples of label values.

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[dimitarvdimitrov]

closing; this interface is only used in mimir, i'll move it there and change it there too