Spectral-index time series over an area

Measure a spectral index over a polygon, scene by scene, back through the Sentinel-2 archive — the answer to "how has this changed since <year>". Costs 0.5 tokens per scene ACTUALLY measured; a scene that fails to read appears in skipped and is not charged. Send estimate_only: true first: that is free and returns how many scenes match, the real date span, and what measuring them would cost. Three limits the response states rather than hides — the archive begins 2015-06-27 so an earlier start is clamped (clamped_to_archive), at most 24 scenes are measured per call so a longer period comes back as a sample with the rest in skipped, and trend compares the first and last measured scene only, so it is not a fitted rate. For a SAR quantity, or to keep measuring as new imagery arrives, create a monitored area instead.