Allow overriding service information from context.

[simitt]

fixes #1175

[graphaelli]

does this mean events can remove service and user information from an event when those are set in the metadata? are there tests for that?

[simitt]

I added some tests showing that data from the metadata.service cannot be deleted but only overriden if set in context.service. This means if there is null set in the event information this does not override the information from the metadata.
One can set an empty string though, in event.context.service as well as in metadata.service. Since we are treating zero values as values, I would not call this deleting. To be consistent and less error prone I think we should either allow zero values on both places or on none.

For users it behaves differently. Since we are not merging the user information but either take the whole information from metadata or from context one could kind of delete information on an event.

I tried to explain this here, but I'll probably have to rephrase as it doesn't seem to be clear enough:

user:

Describes the correlated user for this event. If user data are provided here, all user related information from metadata is ignored, otherwise the metadata's user information will be stored with the event.

service:

Service related information can be sent per event. Provided information will override the more generic information from metadata, non provided fields will be set according to the metadata information.

[eyalkoren]

This is not backported to 6.x, right?

[simitt]

@eyalkoren confirm, it is not backported.

[eyalkoren]

Users with agents that support that upgrading the APM server to 7.x may get their views and aggregations completely changed. Is this documented somewhere?

[simitt]

No, you're right, this is missing a changelog entry. I think this should go into the APM wide release notes and also into the server release notes.

There was kind of a it implicitly works situation though with 6.x I first need to check what really changes for the users when upgrading.