The full JSON for a metadata entry on an Item will be something like
"metadata": {
"alexa": {
"value": "ThermostatController.upperSetpoint"
"config": {
"scale": "Fahrenheit"
}
}
},
The REST API endpoint (assuming it’s still the same in OH 3 (my HestiaPi is offline at the moment)) is a PUT on /items/{itemname}/metadata/{namespace}.
The {itemname} is self explanatory. {namespace} in this case is “alexa”. The JSON to PUT would be
{
"value": "ThermostatController.upperSetpoint"
"config": {
"scale": "Fahrenheit"
}
}