Our hacked-up `_exposition.py` was stripping out some samples it shouldn't
have been. Put them back in, to more closely match the upstream
`exposition.py`.
HTTP requires the response to contain a Content-Length header unless chunked encoding is being used.
Prometheus metrics endpoint did not set this, causing software such as prometheus-proxy to not be able to scrape synapse for metrics.
Signed-off-by: Christian Svensson <blue@cmd.nu>
