Update changelog.md for for exportable metrics

[steve-tsdb]

Describe your changes

Updated changelog for disk/postgresq metrics that are now being or exported or available to be exported.

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• - Is this ready for review? If not, raise as a draft PR
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[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-metrics-export-updates

[atovpeko]

@steve-tsdb I'll merge this for speed's sake and restore the doc link once I've merged the actual doc updates.