[ER] Make `serviceName` configurable via `ELASTIC_APM_SERVICE_NAME` environment variable

[fred-maussion]

Description

The current implementation uses a hardcoded constant for the APM serviceName:

const serviceName = "package-registry"

This prevents dynamic configuration of the APM service name, which can be useful in:

• multi-instance deployments
• containerized environments
• observability use cases (e.g., grouping services by env or cluster)
• debugging or staging environments where custom service naming is required

Expected behavior

The service name used in APM should:

• Default to "package-registry" if the environment variable ELASTIC_APM_SERVICE_NAME is not set
• Use the value from ELASTIC_APM_SERVICE_NAME if present

Proposed implementation

Replace the const with a top-level var initialized with the value of os.Getenv, falling back to the default.

Code snippet:

// Replace this:
const serviceName = "package-registry"

// With this:
var serviceName = getServiceName()

func getServiceName() string {
    if name := os.Getenv("ELASTIC_APM_SERVICE_NAME"); name != "" {
        return name
    }
    return "package-registry"
}

This ensures:

• The env variable is only read once
• The same value is consistently used across APM tracer and logger initialization
• Performance and maintainability are preserved

[fred-maussion]

Closing as part of #1437 PR