Document that range_seconds extends the apparent lifetime of a metric

Related-to: #746

Signed-off-by: Matthias Rampke <[email protected]>

README.md

@@ -83,7 +83,7 @@ resource_id_dimension | Required, under `aws_tag_select`. For the current metric
 aws_statistics | Optional. A list of statistics to retrieve, values can include Sum, SampleCount, Minimum, Maximum, Average. Defaults to all statistics unless extended statistics are requested.
 aws_extended_statistics | Optional. A list of extended statistics to retrieve. Extended statistics currently include percentiles in the form `pN` or `pN.N`.
 delay_seconds | Optional. The newest data to request. Used to avoid collecting data that has not fully converged. Defaults to 600s. Can be set globally and per metric.
-range_seconds | Optional. How far back to request data for. Useful for cases such as Billing metrics that are only set every few hours. Defaults to 600s. Can be set globally and per metric.
+range_seconds | Optional. How far back to request data for. Useful for cases such as Billing metrics that are only set every few hours. Defaults to 600s. Can be set globally and per metric. Note that this will make metrics appear to "live on" for a while in Prometheus when they have stopped being reported in CloudWatch, unless `set_timestamp` is also enabled.
 period_seconds | Optional. [Period](http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/cloudwatch_concepts.html#CloudWatchPeriods) to request the metric for. Only the most recent data point is used. Defaults to 60s. Can be set globally and per metric.
 set_timestamp | Optional. Boolean for whether to set the Prometheus metric timestamp as the original Cloudwatch timestamp. For some metrics which are updated very infrequently (such as S3/BucketSize), Prometheus may refuse to scrape them if this is set to true (see #100). Defaults to true. Can be set globally and per metric.
 use_get_metric_data | Optional. Boolean (experimental) Use GetMetricData API to get metrics instead of GetMetricStatistics. Can be set globally and per metric.