lxd/metrics: OpenMetrics says to end with EOL
lxd/metrics: OpenMetrics says to end with EOL

This allow detecting partial scrapes.

https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#metricfamily-metadata

Signed-off-by: Simon Deziel <eca3f1a14b730e70ce4898287e4fc2f9966fa3a6@canonical.com>

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