promql: Improve the rate extrapolation explanation

Signed-off-by: beorn7 <beorn@grafana.com>

promql/functions.go

@@ -130,10 +130,18 @@ func extrapolatedRate(vals []parser.Value, args parser.Expressions, enh *EvalNod
 	sampledInterval := float64(lastT-firstT) / 1000
 	averageDurationBetweenSamples := sampledInterval / float64(numSamplesMinusOne)
 
-	// If the first/last samples are close to the boundaries of the range,
-	// extrapolate the result. This is as we expect that another sample
-	// will exist given the spacing between samples we've seen thus far,
-	// with an allowance for noise.
+	// If samples are close enough to the (lower or upper) boundary of the
+	// range, we extrapolate the rate all the way to the boundary in
+	// question. "Close enough" is defined as "up to 10% more than the
+	// average duration between samples within the range", see
+	// extrapolationThreshold below. Essentially, we are assuming a more or
+	// less regular spacing between samples, and if we don't see a sample
+	// where we would expect one, we assume the series does not cover the
+	// whole range, but starts and/or ends within the range. We still
+	// extrapolate the rate in this case, but not all the way to the
+	// boundary, but only by half of the average duration between samples
+	// (which is our guess for where the series actually starts or ends).
+
 	extrapolationThreshold := averageDurationBetweenSamples * 1.1
 	extrapolateToInterval := sampledInterval