Not sure about FAA/ICAO.
JAR-OPS 1.297 (a) and (b) use the following wording:
the appropriate weather reports or forecasts or any combination thereof indicate that, during a period commencing 1 hour before and ending 1 hour after the estimated time of arrival at the aerodrome, the weather conditions will be at or above the applicable landing/planning minima
AMC OPS 1.297 uses the following in its table header:
Application of aerodrome forecasts
APPLICATION OF AERODROME FORECASTS (TAF & TREND) TO PRE-FLIGHT PLANNING
Thus there is at least an implicit recognition that METAR trends may be taken into account, but no specific guidance as to how they rank against TAFs.