fix(loki sink): Loki fails to parse protobuf payload with '"' in label value.

[alexgavrushenko]

Summary

When using Loki sink with JSON enconding, label values with '"' and '' chars are pushed to Loki normally, while Protobuf encoding causes errors like: 'couldn't parse labels: ... parse error: unexpected character inside braces' or 'couldn't parse labels: ... parse error: unexpected string'

Vector configuration

Default Vector configuration with Loki sink

How did you test this PR?

There is a unit-test

Change Type

• [x] Bug fix
• [ ] New feature
• [ ] Non-functional (chore, refactoring, docs)
• [ ] Performance

Is this a breaking change?

• [ ] Yes
• [x] No

Does this PR include user facing changes?

• [ ] Yes. Please add a changelog fragment based on our guidelines.
• [x] No. A maintainer will apply the no-changelog label to this PR.

References

N/A

Notes

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