<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def process_header(self, header):
 """ Parse the header for additional metadata. Parameters header : str All the lines in the header. Returns ------- dict The metadata in the header. """

        metadata = {}
        for line in header.split("\n"):
            line = line.strip()
            for tag in self.header_tags:
                tag_text = "{}:".format(tag)
                if line.lower().startswith(tag_text):
                    metadata[tag] = line[len(tag_text) + 1 :].strip()

        return metadata