(Comment about http://hl7.org/fhir/R4/observation-vitalsigns.html#vitals-table)
Such an observation is meaningless in isolation and should be a component, if it can't be accommodated with CIMI extension observation-deviceCode or with Observation.method.
Make temperature device a method, component, or extension (VS ~ https://build.fhir.org/ig/HL7/cimi-vital-signs/ValueSet-heartRateMeasDeviceVS.html)
For Site, use Observation.bodySite
8327-9 - Body temperature measurement site (oral, forehead, rectal, etc.) and 8326-1 -Type of body temperature device MAY be used as additional observations.
(Comment 2 - imported by: Jean Duteau)