New extremities histogram metric breaks metrics

[erikjohnston]

Error is:

AttributeError: 'float' object has no attribute 'replace'
  File "twisted/web/server.py", line 199, in process
    self.render(resrc)
  File "synapse/http/site.py", line 131, in render
    Request.render(self, resrc)
  File "twisted/web/server.py", line 259, in render
    body = resrc.render(self)
  File "twisted/web/resource.py", line 250, in render
    return m(request)
  File "prometheus_client/twisted/_exposition.py", line 18, in render_GET
    return generate_latest(self.registry)
  File "prometheus_client/exposition.py", line 78, in generate_latest
    for k, v in sorted(labels.items())]))
  File "prometheus_client/exposition.py", line 78, in <listcomp>
    for k, v in sorted(labels.items())]))

Local variables at the time: k = 'le', v = 1.0

[hawkowl]

Fixed by #5458