Exponential histograms (and legacy) need to be added to downsampling for completeness. The type remains the same, and the downsampled histogram gets contructed by merging all histograms within each downsample interval.

• Exponential histograms ([Downsampling++] Downsample exponential histograms as a label and metric. #136259)
• TDigest (legacy) histograms (Add TDigest histogram as metric #139247)
• Docs (Add time series histogram to the TSDB docs. docs-content#4548)