## Delta metrics
Metric events where their extent is a time range are deltas. Their `metric_value` is the result of applying their `metric_agg` over the underlying stream within the extent.
The following metric reports that the number of bytes written changed by 17 over the last 30 seconds:
```rust
# extern crate emit;
let now = emit::clock().now();
let last_sample = now.map(|now| now - std::time::Duration::from_secs(30));
emit::emit!(
extent: last_sample..now,
"{metric_agg} of {metric_name} is {metric_value}",
evt_kind: "metric",
metric_agg: "count",
metric_name: "bytes_written",
metric_value: 17,
);
```
```text
Event {
mdl: "my_app",
tpl: "{metric_agg} of {metric_name} is {metric_value}",
extent: Some(
"2024-04-30T06:55:59.839770000Z".."2024-04-30T06:56:29.839770000Z",
),
props: {
"evt_kind": metric,
"metric_name": "bytes_written",
"metric_agg": "count",
"metric_value": 17,
},
}
```