Support overriding the diagnostics service name

[tostavio]

Summary

This pull request lets each ConfigCat Proxy deployment configure the OpenTelemetry service name used for diagnostics telemetry.

Motivation

Multiple ConfigCat Proxy deployments can run in the same Kubernetes cluster or observability environment. A fixed service name makes their OTLP metrics and traces appear under the same service.name resource attribute, which makes monitoring and filtering ambiguous.

This setting lets operators assign a distinct service name per deployment, for example when different proxies serve different environments, tenants, network zones, or traffic paths.

Fix

The key behavior change is in the telemetry reporter. The reporter now builds the OpenTelemetry resource from the configured diagnostics service name instead of always writing the fixed value:

- res := buildResource(version)
+ res := buildResource(conf.ServiceName, version)

- semconv.ServiceName("configcat-proxy"),
+ semconv.ServiceName(serviceName),

The YAML and environment configuration changes only provide this value through the existing diagnostics configuration path.

Changes

• Adds diag.service_name to the YAML configuration.
• Adds CONFIGCAT_DIAG_SERVICE_NAME as the matching environment override.
• Uses the configured value for the OpenTelemetry service.name resource attribute.

Compatibility

When no service name is configured, ConfigCat Proxy keeps using configcat-proxy.

Testing

• go fmt ./...
• go vet -tags testing ./...
• go test -tags testing -race ./config -run TestDiagConfig
• go test -tags testing -race ./diag/telemetry -run TestBuildResource_ServiceName

Closes #54.

[z4kn4fein]

Hi @tostavio, thank you for your contribution!