Cloud cost
Metric cardinality
The number of unique metric-name + tag-value combinations a monitoring platform has to store and bill for, not the number of metric names your code emits.
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Definition
A single metric.submit call in code can become thousands of billable series once cardinality is counted correctly: cardinality is the count of every distinct combination of a metric's name and its tag values, including the host tag on most platforms. Tagging one metric by customer_id or request_id across a growing user base multiplies it into a five- or six-figure billable count without a single new metric name ever being created.
Histogram and distribution metric types multiply cardinality further, since each one bills as several underlying metrics (commonly max, median, avg, a percentile, and count) per unique tag combination by default.
Where it shows up
Metric cardinality shows up in a monitoring platform's own usage dashboard as a billable series count, distinct from the number of metric names a codebase actually emits in its source. Datadog and similar platforms report this count directly, including the host tag most platforms add to every series automatically.
What makes it expensive
A common expensive habit is tagging a metric by a high-cardinality field like customer_id or request_id across a growing user base, multiplying one metric name into a five- or six-figure billable series count. This does not creep. It steps, on the day the tag ships, which makes a cardinality report worth reading straight after any tagging change.
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