Update spec/index.md

Co-authored-by: David Derler <[email protected]>

spec/index.md

@@ -2103,7 +2103,9 @@ The node metrics management canister API is considered EXPERIMENTAL. Canister de
 
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-Given a subnet ID as input, this method returns collections of metrics for all nodes on the given subnet (field `node_metrics`) at timestamps provided in nanoseconds since 1970-01-01 (field `timestamp_nanos`) at which the metrics were sampled. The returned timestamps are all timestamps after (and including) the provided timestamp (field `start_at_timestamp_nanos`) for which node metrics are available. The maximum number of returned timestamps is 60 and no two returned timestamps belong to the same UTC day.
+Given a subnet ID as input, this method returns a time series of node metrics (field `node_metrics`). The timestamps are represented as nanoseconds since 1970-01-01 (field `timestamp_nanos`) at which the metrics were sampled. The returned timestamps are all timestamps after (and including) the provided timestamp (field `start_at_timestamp_nanos`) for which node metrics are available. The maximum number of returned timestamps is 60 and no two returned timestamps belong to the same UTC day.
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+Note that a sample will only include metrics for nodes whose metrics changed compared to the previous sample. This means that if a node disappears in one sample and later reappears its metrics will restart from 0 and consumers of this API need to adjust for these resets when aggregating over multiple samples.
 
 A single metric entry is a record with the following fields: