Docs on what are valid field and tag key names

We do not have (or at least I could not find) any docs on what characters we allow as field and tag keys.

The answer depends on a combination of both line protocol and Flux as those are the systems used to write and read the data.

Flux allows for any valid printable utf-8 sequence for a field or tag key. The syntax is `r["any string with dots. or other utf-8 chars"
]`. For line protocol I believe it is the same but I would double check with @rogpeppe.

It would be helpful to have a single place where this is discussed directly so users do not have to do the investigation themselves of the detailed specs for both line protocol and Flux. For example here are the relevant bits for Flux https://docs.influxdata.com/influxdb/cloud/reference/flux/language/expressions/#member-expressions