filebeat/filestream: Use gzip.Reader.Reset instead of Close+NewReader

[orestisfl]

Proposed commit message

filebeat/filestream: Use gzip.Reader.Reset instead of Close+NewReader

Replace the pattern of closing the gzip reader and creating a new one with a call to gzip.Reader.Reset() in gzipSeekerReader.Seek().

Reset() is the idiomatic way to reuse a gzip reader when switching to a new underlying reader. It:

• Reuses internal buffers instead of allocating new ones, reducing memory pressure and GC overhead
• Is semantically equivalent to creating a new reader (discards current state and reinitializes)
• Eliminates the unnecessary Close() call whose error was being ignored anyway

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

Run the gzip-related unit and integration tests

Related issues

• I discovered this while reviewing filebeat: filestream input implements lazy file opening during directory scans #48506. However, if I am not mistaken, it does not lead to less open syscalls.

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