private void injectUserMetadata(Metadata userMetadata, List<Metadata> metadataList) {
        for (String n : userMetadata.names()) {
            //overwrite whatever was there
            metadataList.get(0).set(n, null);
            for (String val : userMetadata.getValues(n)) {
                metadataList.get(0).add(n, val);
            }
        }
    }
returns stack trace if there was a container exception or empty string
if there was no stacktrace

@param t
@param metadataList
@return
tika-core/src/main/java/org/apache/tika/pipes/PipesServer.java