Allow ingester.Push() to proceed on soft errors only

[duricanikolic]

What this PR does

Ingester's ingester.pushSamplesToAppender(), called by ingester.Push(), appends samples and exemplars to the appender. The former distinguishes between:

• soft errors, which should allow ingester.Push() to continue its execution and to ingest all valid samples and exemplars, and to return the first encountered error only at the end, and
• other errors, that should be returned immediately.

So far there was no precise way to determine which error is a soft error. This PR introduces softError, an interface that should be implemented by all the errors that shouldn't prevent ingester.Push() from going ahead. Only these errors will be considered as soft errors by pushSamplesToAppender().

Which issue(s) this PR fixes or relates to

Related to #6008

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

Thanks. I'm not sure about terminology, but we can change that in the future if we have a better idea.

[duricanikolic]

Thanks. I'm not sure about terminology, but we can change that in the future if we have a better idea.

I used that name (softError) because these errors were already called like that in the present codebase. We will change it later if we find a better idea.