Requesting badly formatted PVs breaks metrics parsing

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...