[enrichment] Check for attributes first to prevent overriding the attributes

The enrichment processor adds Elastic attributes without checking if the attributes exist. Since the enrichments are derived from OTel attributes and are required for the APM UIs to work, this seems to be a safe option. However, it might also be disruptive if we end up overriding existing attributes set by the user or attributes set manually while instrumenting. Consider these cases and handle overrides as required.