test: add metadata process tests and increase coverage

[kruskall]

Add testcases for uncompressed, gzip and deflate. Ensure an
error is returned if the data is malformed and increase
code coverage.
Bring coverage to 100% for metadata.go

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-08-08T17:13:04.371+0000

• Duration: 6 min 16 sec

Test stats 🧪

Test	Results
Failed	0
Passed	130
Skipped	32
Total	162

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[axw]

LGTM!