Fix format of line protocol metrics syntax

[satta]

:notebook_with_decorative_cover: Description

The current implementation of the metrics value serialization includes spaces into the fields of the measurements, which is not valid according to the spec and will cause the measurement to be rejected. This PR changes this behaviour to ensure all fields are separated by commas.

:memo: Checklist

• [X] All user-facing changes have changelog entries.
• [X] The changes are reflected on docs.tenzir.com/threatbus, if necessary.
• [X] The PR description contains instructions for the reviewer, if necessary.

:dart: Review Instructions

Compare code and output to line protocol specification.

[satta]

Can you please add a changelog entry in apps/vast/CHANGELOG.md?

Done!