Environment Variable Substitution Broken in Prometheus Receiver Due to Over-escaping

## Summary
Environment variable substitution (e.g., `${env:VARIABLE_NAME}`) in Prometheus receiver `metric_relabel_configs` and `relabel_configs` is broken due to incorrect automatic escaping of `$` characters, resulting in literal strings instead of variable substitution.

## Environment
- **Ops Agent Version**: [Latest from master branch]
- **Platform**: Linux
- **OpenTelemetry Collector**: otelopscol (ops-agent embedded)

## Expected Behavior
When using environment variable substitution syntax in Prometheus receiver configuration:

```yaml
metrics:
receivers:
prometheus:
type: prometheus
config:
scrape_configs:
- job_name: 'triton'
scrape_interval: 60s
static_configs:
- targets: ['triton.service:8002']
metrics_path: /metrics
metric_relabel_configs:
- source_labels: [__name__]
regex: 'nv_inference_count|nv_gpu_utilization|nv_gpu_memory_total_bytes|nv_gpu_memory_used_bytes'
action: keep
- action: replace
replacement: ${env:ENVIRONMENT}
target_label: environment
```

The generated OpenTelemetry collector configuration should contain:
```yaml
replacement: ${env:ENVIRONMENT}
```

And the collector should substitute the actual environment variable value.

## Actual Behavior
The generated OpenTelemetry collector configuration contains:
```yaml
replacement: $${env:ENVIRONMENT}
```

According to OTel documentation, `$$` indicates a literal `$`, so this results in the literal string `${env:ENVIRONMENT}` being used as the label value instead of the environment variable's actual value.

## Root Cause
In [`confgenerator/prometheus.go` lines 123-127](https://github.com/GoogleCloudPlatform/ops-agent/blob/master/confgenerator/prometheus.go#L123-L127), the code unconditionally escapes all `$` characters:

```go
// Escape the $ characters in the regexes.
for i := range copyPromConfig.ScrapeConfigs {
for j := range copyPromConfig.ScrapeConfigs[i].RelabelConfigs {
rc := copyPromConfig.ScrapeConfigs[i].RelabelConfigs[j]
rc.Replacement = strings.ReplaceAll(rc.Replacement, "$", "$$") // ← Problem here
}
for j := range copyPromConfig.ScrapeConfigs[i].MetricRelabelConfigs {
mrc := copyPromConfig.ScrapeConfigs[i].MetricRelabelConfigs[j]
mrc.Replacement = strings.ReplaceAll(mrc.Replacement, "$", "$$") // ← Problem here
}
}
```

## Steps to Reproduce

1. Create Ops Agent config with environment variable substitution:
```yaml
metrics:
receivers:
prometheus:
type: prometheus
config:
scrape_configs:
- job_name: 'test'
metric_relabel_configs:
- action: replace
replacement: ${env:TEST_VAR}
target_label: test_label
service:
pipelines:
custom_metrics_pipeline:
receivers: [prometheus]
```

2. Set environment variable: `export TEST_VAR="expected_value"`

3. Generate OTel config: `./libexec/google_cloud_ops_agent_engine -service otel`

4. Check generated `otel.yaml` - you'll see:
```yaml
replacement: $${env:TEST_VAR}
```

5. Run the collector - labels will contain literal `${env:TEST_VAR}` instead of `expected_value`

## Proposed Solution

The escaping logic should distinguish between:
1. **Regex capture groups** (e.g., `$1`, `$2`) - these should be escaped to `$$1`, `$$2`
2. **Environment variable substitution** (e.g., `${env:VAR}`) - these should NOT be escaped

Suggested fix:

```go
// Escape only regex capture groups, not environment variables
for i := range copyPromConfig.ScrapeConfigs {
for j := range copyPromConfig.ScrapeConfigs[i].RelabelConfigs {
rc := copyPromConfig.ScrapeConfigs[i].RelabelConfigs[j]
rc.Replacement = escapeRegexCaptureGroups(rc.Replacement)
}
for j := range copyPromConfig.ScrapeConfigs[i].MetricRelabelConfigs {
mrc := copyPromConfig.ScrapeConfigs[i].MetricRelabelConfigs[j]
mrc.Replacement = escapeRegexCaptureGroups(mrc.Replacement)
}
}

func escapeRegexCaptureGroups(s string) string {
// Escape $1, $2, etc. but not ${env:...}
re := regexp.MustCompile(`\$(\d+)`)
return re.ReplaceAllString(s, "$$${1}")
}
```

## Impact
- Environment variable substitution is completely broken in Prometheus receiver
- Users cannot dynamically configure labels based on environment
- Workaround requires manual editing of generated `otel.yaml` files

## Related Files
- `confgenerator/prometheus.go` (lines 123-127)
- Comment at lines 48-52 mentions the escaping but doesn't account for env vars

## Additional Context
The original comment suggests this escaping was added for regex capture groups, but it's too broad and breaks legitimate environment variable usage that OpenTelemetry collector supports.