# Regarding Metric Filter
**URL:** https://discuss.elastic.co/t/regarding-metric-filter/265211
**Category:** Logstash
**Created:** [February 23, 2021, 2:30pm UTC](https://discuss.elastic.co/t/regarding-metric-filter/265211 "2021-02-23T14:30:09Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@dawiro](https://discuss.elastic.co/u/dawiro)
#### Post date: [February 23, 2021, 2:30pm UTC](https://discuss.elastic.co/t/regarding-metric-filter/265211/1 "2021-02-23T14:30:10Z")
Hi,
I'm trying to dynamically add fields to a metric filter. Can you tell me why this is failing interpolation?
```auto
metrics {
meter => ["events"]
add_field => {
"agent" => "%{[agent][hostname]}"
}
add_tag => ["metric"]
enable_metric => false
}
```
if I use the same syntax with the mutate filter it works fine:
```auto
mutate {
add_field => {
"agent" => "%{[agent][hostname]}"
}
}
```
Logstash version is 7.11.1.
Thx
D
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [February 23, 2021, 3:50pm UTC](https://discuss.elastic.co/t/regarding-metric-filter/265211/2 "2021-02-23T15:50:44Z")
> [@dawiro](#):
>
> I'm trying to dynamically add fields to a metric filter. Can you tell me why this is failing interpolation?
The metrics filter is generating a new event based on the number of events it has seen in the previous five seconds. If it has seen 10 events, from which of those should it extract [agent][hostname]? logstash cannot decide. See [issue 49](https://github.com/logstash-plugins/logstash-filter-metrics/issues/49).
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [March 23, 2021, 3:51pm UTC](https://discuss.elastic.co/t/regarding-metric-filter/265211/3 "2021-03-23T15:51:35Z")
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