* add labelallow stage

* added docs

* add docs

* fix lint

* update stage name in _index.md

(cherry picked from commit 0f5fcee)

* Fixes a bug where unpack would mutate log line.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* add reset.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
(cherry picked from commit 6148794)

When splitting correctly steps in the frontend, some queries might be of a single step, e.g start == end.

This PR add supports for those queries, which are already supported by Cortex and Prometheus, but for some reason not Loki.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
(cherry picked from commit 29a3240)

* Fixes a bug in MatrixStepper when sharding queries.

Since we split correctly metric queries, this bug has show itself.
Basically we were not correctly stepping through time. We should always start from start, add the step until the start is after the end.

For more read: https://www.robustperception.io/step-and-query_range

Fixes #3541

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Fixes tests.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Fixes tests. for real.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* Not easy to get those test working.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
(cherry picked from commit b11d2ef)

I think this was only impacting the docker driver who would start/stop for each new followed containers.
My assumption is that this would slowly build up and eat CPU over time.

Fixes #3319

I arrived to this conclusion since strace was showing a crazy amount of futex syscall and nothing else seems to be the cause.

:pray:

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>
(cherry picked from commit ecca5d3)

* Fixes a race when using specific tenant and multi-client.

This was because each client would try to mutate the original set of labels.
Instead of cloning the map which can be expensive, I created a little helper that generate the string
from the labelset without a set of label name.

Fixes #3571

I also added a test to see if this was reproducible and it was indeed:

```
~/go/src/github.com/grafana/loki master*
❯ go test -timeout 30s -tags dev,gofuzz -race -run ^TestMultiClient_Handle_Race$ github.com/grafana/loki/pkg/promtail/client -v -count=1 -timeout=0s
=== RUN   TestMultiClient_Handle_Race
==================
WARNING: DATA RACE
Read at 0x00c0000b77d0 by goroutine 22:
  runtime.mapaccess2_faststr()
      /usr/local/Cellar/go/1.16.2/libexec/src/runtime/map_faststr.go:107 +0x0
  github.com/grafana/loki/pkg/promtail/client.(*client).getTenantID()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:376 +0xcb
  github.com/grafana/loki/pkg/promtail/client.(*client).processEntry()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:407 +0x9a
  github.com/grafana/loki/pkg/promtail/client.(*client).run()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:235 +0x36a

Previous write at 0x00c0000b77d0 by goroutine 21:
  runtime.mapdelete_faststr()
      /usr/local/Cellar/go/1.16.2/libexec/src/runtime/map_faststr.go:297 +0x0
  github.com/grafana/loki/pkg/promtail/client.(*client).processEntry()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:408 +0x144
  github.com/grafana/loki/pkg/promtail/client.(*client).run()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:235 +0x36a

Goroutine 22 (running) created at:
  github.com/grafana/loki/pkg/promtail/client.New()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:198 +0x7dc
  github.com/grafana/loki/pkg/promtail/client.TestMultiClient_Handle_Race()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/multi_test.go:126 +0x2c7
  testing.tRunner()
      /usr/local/Cellar/go/1.16.2/libexec/src/testing/testing.go:1194 +0x202

Goroutine 21 (running) created at:
  github.com/grafana/loki/pkg/promtail/client.New()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/client.go:198 +0x7dc
  github.com/grafana/loki/pkg/promtail/client.TestMultiClient_Handle_Race()
      /Users/ctovena/go/src/github.com/grafana/loki/pkg/promtail/client/multi_test.go:124 +0x187
  testing.tRunner()
      /usr/local/Cellar/go/1.16.2/libexec/src/testing/testing.go:1194 +0x202
==================
    testing.go:1093: race detected during execution of test
--- FAIL: TestMultiClient_Handle_Race (0.00s)
=== CONT
    testing.go:1093: race detected during execution of test
FAIL
FAIL	github.com/grafana/loki/pkg/promtail/client	0.022s
FAIL
```

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

* don't overload variable name

Co-authored-by: Edward Welch <edward.welch@grafana.com>
(cherry picked from commit 0107a11)