Requesting badly formatted PVs breaks metrics parsing

[AlexanderWells-diamond]

If you request a PV using a clearly incorrect syntax, e.g. CALC\\\{(A%2)\}(BL05J-VA-IOC-02:HEARTBEAT), this string will appear as-is in the metrics overview. This particular string breaks the parsing of the metrics completely, and so no Coniql metrics appear in Prometheus.

We will need to do some escaping of strings. There may be functionality in aioprometheus for such behaviour. I'm sort of surprised it doesn't automatically escape strings...

[AlexanderWells-diamond]

I think aioprometheus should do some validation of this - I raised claws/aioprometheus#91.

We can still apply our own escaping to the labels before passing them on.