compact: relax sorted label names check under accept-malformed-index flag

[bwplotka]

Also in "relax" mode, we should produce metrics.

Relevant discussion: #848 (comment)

Acceptance Criteria:

• Add debug.accept-malformed-index flag.
• When this flag is true, relax the compaction index verification to not check if label names are sorted and produce metrics when this issue occurs so that it'd be known.

[GiedriusS]

Fixed an issue upstream which makes Prometheus produce invalid metrics with upper-case letters in label names: prometheus/prometheus#5372
The debug.accept-malformed-index flag should make Thanos Compact ignore the fact that the invariant is not being upheld i.e. it should turn that check off mentioned in #848.

[GiedriusS]

Also, perhaps this should be called --ignore-nonsorted-labels.

[GiedriusS]

A hidden --debug.accept-malformed-index flag has been added to Thanos Compactor recently, courtesy of @jjneely. Thanks a lot! So now this has been fixed in Prometheus >2.8.0, and if users are running older Prometheus versions they can enable this (which is mentioned in the error message) to ignore this error since it is inevitable if any of the metrics have upper-case label names.