`limit` keyword does not work with the `hidden` keyword

`limit` does not limit the hidden metrics.

Here is my test:

gen_log.sh

```sh
#!/bin/bash

for i in {1..100}
do
echo id=$i >> test.log
done
```

Execute gen_log.sh will generate 100 lines with sequence IDs, and write to the file test.log.

test.mtail

```
counter test_total by id limit 10

/id=(?P\d+)/ {
test_total[$id] ++
}

```

test_hidden.mtail

```
hidden counter test_total by id limit 10

/id=(?P\d+)/ {
test_total[$id] ++
}

```

Both are the same except for the `hidden` prefix.

start mtail:

```
./mtail -expired_metrics_gc_interval=30s -logtostderr -mtailDebug -v 10 --progs test.mtail --logs test.log
```

After 30 seconds, when the GC occurs, I see the following message:

```
I0422 15:58:35.501660 40454 filestream.go:182] &{0xc000181aa0}: current seek is 1184
I0422 15:58:35.501682 40454 filestream.go:183] &{0xc000181aa0}: new size is 1184
I0422 15:58:35.501600 40454 metric.go:192] removeOldest: removing oldest LV: &{[1] 0xc000113030 0s}
I0422 15:58:35.501695 40454 filestream.go:243] &{0xc000181aa0}: waiting
I0422 15:58:35.501709 40454 metric.go:192] removeOldest: removing oldest LV: &{[2] 0xc000113050 0s}
I0422 15:58:35.501725 40454 metric.go:192] removeOldest: removing oldest LV: &{[3] 0xc000113070 0s}
I0422 15:58:35.501737 40454 metric.go:192] removeOldest: removing oldest LV: &{[4] 0xc0001130a0 0s}
I0422 15:58:35.501748 40454 metric.go:192] removeOldest: removing oldest LV: &{[5] 0xc0001aa020 0s}
...
I0422 15:58:35.502735 40454 metric.go:192] removeOldest: removing oldest LV: &{[88] 0xc0001aa4c0 0s}
I0422 15:58:35.502746 40454 metric.go:192] removeOldest: removing oldest LV: &{[89] 0xc0001aa4e0 0s}
I0422 15:58:35.502756 40454 metric.go:192] removeOldest: removing oldest LV: &{[90] 0xc0001aa530 0s}
```

But when using test_hidden.mtail, the above message is not displayed.