autopkgtest flaky in hirsute: goleak: Errors on successful test run: found unexpected goroutines:

[Balint Reczey]

https://autopkgtest.ubuntu.com/packages/prometheus/hirsute/s390x

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-hirsute/hirsute/s390x/p/prometheus/20210104_200329_3b655@/log.gz

...
--- PASS: TestMigrateWAL_Fuzz (0.00s)
PASS
goleak: Errors on successful test run: found unexpected goroutines:
[Goroutine 7079 in state chan send, with github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func1 on top of the stack:
goroutine 7079 [chan send]:
github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func1(0xc00009b6b0, 0xc00000eea0)
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:570 +0x7a
created by github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:569 +0x70

 Goroutine 7080 in state chan send, with github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func2 on top of the stack:
goroutine 7080 [chan send]:
github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func2(0xc00009b6b0)
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:599 +0x74
created by github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:598 +0x116

 Goroutine 7538 in state chan send, with github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func2 on top of the stack:
goroutine 7538 [chan send]:
github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut.func2(0xc002fe4840)
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:599 +0x74
created by github.com/prometheus/prometheus/tsdb.(*SegmentWAL).cut
 /tmp/autopkgtest.nCbU2V/autopkgtest_tmp/build/src/github.com/prometheus/prometheus/tsdb/wal.go:598 +0x116
]
FAIL github.com/prometheus/prometheus/tsdb 43.718s
...

There is an open upstream pull request for fixing it linked from the upstream issue:
https://github.com/prometheus/prometheus/issues/7672