Kibana Alerts to index is giving me a JSON with key/value instead of just the value

Kibana version 8.17.0

Hi, I set up an index to store the alerts, Im using metric threshold and when I use the variable {{context.metric}} y get a JSON instead of just the value of the metric:

{"condition0":"gcp.compute.instance.cpu.usage.pct"}

Is this the normal behavior? or I am doing something wrong?

this is my document:

{
"dataset": "gcp.compute",
"fuente": "{{context.group}}",
"razon": "{{context.reason}}",
"metrica": "{{context.metric}}",
"estado": "{{context.alertState}}",
"@timestamp": "{{context.timestamp}}",
"ctx_cloud": "{{context.cloud}}",
"ctx_host": "{{context.host}}"
}