[chore][cmd/mdatagen] Properly test bytes attribute type (#9229)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
As described in the [issue opened in the contrib
repository](open-telemetry/opentelemetry-collector-contrib#29923),
mdatagen was originally generating tests for byte arrays incorrectly.
This tests byte arrays in their proper format.

As
[requested](open-telemetry/opentelemetry-collector-contrib#30106 (comment)),
this ports the [contrib fix
](open-telemetry/opentelemetry-collector-contrib#30106
)into core.

**Link to tracking Issue:** <Issue number if applicable>

open-telemetry/opentelemetry-collector-contrib#29923

**Testing:** <Describe what testing was performed and which tests were
added.>
I added a fake attribute and fake metric that used it, then generated
tests.
Fake attribute and metric:
```
attributes:
  # Added a fake attribute in a metadata.yaml file
  fake:
    description: "Fake attribute for testing"
    type: bytes

metrics:
  container.cpu.usage.percpu:
    enabled: false
    description: "Per-core CPU usage by the container (Only available with cgroups v1)."
    unit: ns
    sum:
      value_type: int
      monotonic: true
      aggregation_temporality: cumulative
    attributes:
      - fake
```
Generated tests:
```
// First test reference:
			mb.RecordContainerCPUUsagePercpuDataPoint(ts, 1, []byte("fake-val"))
// Second test reference:
					attrVal, ok := dp.Attributes().Get("fake")
					assert.True(t, ok)
					assert.EqualValues(t, []byte("fake-val"), attrVal.Bytes())
```

cmd/mdatagen/loader.go

@@ -194,7 +194,7 @@ func (a attribute) TestValue() string {
  case pcommon.ValueTypeSlice:
  return fmt.Sprintf(`[]any{"%s-item1", "%s-item2"}`, a.FullName, a.FullName)
  case pcommon.ValueTypeBytes:
- return fmt.Sprintf(`bytes("%s-val")`, a.FullName)
+ return fmt.Sprintf(`[]byte("%s-val")`, a.FullName)
  }
  return ""
 }