fix(loki.process): Multiline no longer pass empty entry if start was flushed

[kalleep]

Issue(s) fixed by this Pull Request

In #5702 some changes to stage.multiline was made to support created timestamp along with some optimizations (No longer do a deep clone). But we also made sure to always reset to a zero entry.

This is a issue when a multiline entry is flushed before completion, either by max line or max wait. Then all lines that is not considered start line will be added to an empty entry and later flushed.

To fix this we revert to the old behavior where we clone the stored entry and keep it around for other lines.

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated

[Copilot]

Pull request overview

Fixes stage.multiline behavior where continuation lines could be appended to/flush as an empty entry after the start line was flushed early (max lines / max wait), by preserving the start-line entry metadata across flushes.

Changes:

• Preserve startLineEntry across multilineState.Reset() to keep prior start-line context for subsequent non-start lines.
• Rebuild flushed entries by cloning labels/extracted/structured metadata while preserving the original created timestamp.
• Add a regression test covering “start line flushed before new” behavior and adjust test logger usage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/component/loki/process/stages/multiline.go	Keeps start-line context after flush and constructs flushed entries with cloned metadata + preserved created timestamp.
internal/component/loki/process/stages/multiline_test.go	Updates logger setup and adds a regression test for early flush behavior.