Update enrichment to handle existing attributes

[isaacaflores2]

Overview

Traditional APM event ingestion utilized enrichment from the apm-data lib. With the introduction of the receiver/elasticapmintake component (which is using apm-data internally) it is possible for a collector to have a pipeline such as: receiver/elasticapmintake -> processor/elasticapm.

The processor/elasticapm and the opentelemetry-lib should account for this by not overriding any pre-existing elastic attributes. This will preserve the existing values and avoid any unintended changes to elastic attributes

This is a follow up to #237

Changes

• Added attribute.IsEmpty helper function which checks if an attribute exists or is empty. I opted to use this approach since in some it allows the logic to exit early when the attribute exists. The downside is that it adds a lot more if statements everywhere.
  • The alternative would be to add a wrapper around Attributes().PutStr but this would require the enrichment logic to always attempt to derive the new value which would be a little less efficient.
• Updated all enrichment logic to only set an attribute if it does not exists or is empty
  • The logic around transaction.type required special consideration since the library itself can set the transaction.type
• Updated isElasticTransaction to check processor.event. This avoids having to derive if a span is an elastic transaction when it is already explicitly considered a transactions. It also avoids reclassifying a span to a transaction or a transaction to a span.