Fix value counts on boolean and string charts

On measurements with more than 500 records, the boolean and string control charts derived their value distribution from a sampled subset, so the True/False split and per-value counts could be slightly off. They now reflect the exact full dataset.

• Fixed the True/False and string value counts to be computed over all records, not the chart's sampled points.
• Kept the chart fast by computing the exact distribution server-side only when sampling is in effect.