[exporter/file] Omit length-prefix framing for stream-decodable encodings when native compression is enabled

[paulojmdias]

Description

Under the exporter.file.nativeCompression feature gate, output from a custom encoding extension was always length-prefixed (exportMessageAsBuffer), so text_encoding files decompressed to a 4-byte prefix + text instead of clean text. Now, encodings that are stream-decodable on read-back (implement encoding.LogsDecoderFactory, e.g., text_encoding) are written in newline-delimited format. Proto and other encodings keep length-prefix framing.

Link to tracking issue

Fixes #49328

Testing

Added unit tests: stream-decodable encoding yields clean newline-delimited output (no prefix), and verified end-to-end with text_encoding + zstd + the gate, and zstdcat output is now clean text.

Documentation

Updated native-compression note.

Authorship

• I, a human, wrote this pull request description myself.

[Copilot]

Pull request overview

Updates the fileexporter native-compression path to avoid 4-byte length-prefix framing for stream-decodable log encodings (e.g., text_encoding), so decompressed .zst output is clean, newline-delimited text suitable for standard tooling.

Changes:

• Thread an encodingUnframed capability from the selected encoding extension into the file writer’s framing selection under exporter.file.nativeCompression.
• Adjust native-compression framing selection to use newline-delimited output for eligible encodings (and keep length-prefix framing for proto/other encodings).
• Add unit tests validating unframed/newline-delimited behavior for stream-decodable encodings, plus documentation and changelog updates.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
exporter/fileexporter/file_writer.go	Adds encodingUnframed parameter to framing selection to choose newline-delimited output under native compression.
exporter/fileexporter/marshaller.go	Detects stream-decodable encoding extensions (via encoding.LogsDecoderFactory) and records the capability.
exporter/fileexporter/file_exporter.go	Passes encodingUnframed into framing selection for the standard exporter.
exporter/fileexporter/grouping_file_exporter.go	Passes encodingUnframed into framing selection for the grouping exporter.
exporter/fileexporter/native_compression_test.go	Adds tests validating unframed behavior for stream-decodable encodings and framing toggle behavior.
exporter/fileexporter/file_exporter_test.go	Updates tests for the new buildExportFunc signature.
exporter/fileexporter/README.md	Documents newline-delimited output for JSON and stream-decodable encodings under native compression.
exporter/fileexporter/go.mod	Promotes extension/encoding to a direct dependency due to new imports.
.chloggen/fileexporter-unframed-text-encoding.yaml	Changelog entry for the framing behavior change under the feature gate.

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