[bug-hunter] metricbeat openmetrics label prefix remover panics on short keys

## Impact
`metricbeat/helper/openmetrics` can panic at runtime when removing label key prefixes, crashing processing for valid label maps. Any caller using `OpLabelKeyPrefixRemover` with a prefix shorter than 6 can trigger a deterministic `slice bounds out of range` panic when a label key length is between `len(prefix)` and `5`.

## Reproduction Steps
1. From repo root, create and run this minimal repro:

```go
// /tmp/gh-aw/agent/repro_openmetrics_panic.go
package main

import (
"github.com/elastic/elastic-agent-libs/mapstr"
"github.com/elastic/beats/v7/metricbeat/helper/openmetrics"
)

func main() {
labels := mapstr.M{"ex_j": "v"}
openmetrics.OpLabelKeyPrefixRemover("ex_").Process("", nil, labels)
}
```

2. Run:

```bash
go run /tmp/gh-aw/agent/repro_openmetrics_panic.go
```

## Expected vs Actual
**Expected:** Prefix-removal should compare using prefix length and return normally.

**Actual:** Process panics:

```text
panic: runtime error: slice bounds out of range [:6] with length 4

goroutine 1 [running]:
github.com/elastic/beats/v7/metricbeat/helper/openmetrics.opLabelKeyPrefixRemover.Process(...)
/home/runner/work/beats/beats/metricbeat/helper/openmetrics/metric.go:487
main.main()
/tmp/gh-aw/agent/repro_openmetrics_panic.go:12 +0x149
exit status 2
```

## Failing Test
```go
package main

import (
"github.com/elastic/elastic-agent-libs/mapstr"
"github.com/elastic/beats/v7/metricbeat/helper/openmetrics"
)

func main() {
labels := mapstr.M{"ex_j": "v"}
openmetrics.OpLabelKeyPrefixRemover("ex_").Process("", nil, labels)
}
```

## Evidence
- `metricbeat/helper/openmetrics/metric.go:484-488` currently does:
- `if len(k) < len(o.Prefix) { continue }`
- `if k[:6] == o.Prefix { ... }` ← fixed-width slice causes panic for short keys.
- The equivalent logic in prometheus was fixed to dynamic slicing in `metricbeat/helper/prometheus/metric.go:437-441`:
- `if k[:len(o.Prefix)] == o.Prefix { ... }`
- Recent fix commit `ff87e683af624c4bdac64602a36429080f90614b` addressed this class of bug in prometheus path but not openmetrics.

---
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