Remove trailing backslash from tag keys/values

[danielnelson]

Currently a tag key or value with a trailing backslash will produce an error like:

 E! [outputs.influxdb] When writing to [http://localhost:8086]: received error unable to parse 'mem,host=loaner,path=C:\ free=3166146560i 1591645472000000000': invalid tag format; discarding points

With this change the trailing slashes are stripped. Other methods were considered include using a space or a replacement char such as _. I decided not to use a space since it might cause confusion when writing queries by hand and in the cli. I decided against underscore because due to how ugly it is.

I know ideally we would have some form of influxdata/influxdb#6037, but I believe this needs to be worked out separately from a fix for the current line protocol.

closes #7558

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[reimda]

This needs to be documented. I expected a note in the serializer readme and maybe links in the output plugins readmes to see details in the serializer readme. I took a quick look and didn't find anything about float nan and inf being dropped either.

[danielnelson]

I added documentation about these corner cases.