Try this xpath config:
[[inputs.http.xpath]]
metric_name = "'hygrometer'"
metric_selection = "/stateList/device[@name='Hygrometer_LR']/channel[@ise_id='1576']"
timestamp = "datapoint[@type='TEMPERATURE']/@timestamp"
timestamp_format = "unix"
[inputs.http.xpath.tags]
room = "string(../@name)"
[inputs.http.xpath.fields]
humidity = "number(datapoint[@type='HUMIDITY']/@value)"
temperature = "number(datapoint[@type='TEMPERATURE']/@value)"