fix: Add copyString function to symbolizer to avoid retaining memory

[cyriltovena]

What this PR does / why we need it:

This was reported by the community that when using structured metadata ingester were leaking memory.

Labels have a special proto Unmarshaller to avoid string copy since it was never interned before it was never causing a problem. But since the addition of structured metadata we now do intern thoses labels and they hold reference of the full GRPC buffer from the push request.

https://github.com/grafana/loki/blob/main/vendor/github.com/grafana/loki/pkg/push/types.go#L667

To fix the issue I simply copy the string before interning it. We could consider removing this optimisation too, but I'm afraid it will have other impact so going safe for now.

Which issue(s) this PR fixes:
Fixes #13123

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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