Backend granularity (service.target.*) clarifications

# Description

Spec PR https://github.com/elastic/apm/pull/674 ([the diff](https://github.com/elastic/apm/pull/674/files)) makes some updates and clarifications to the specs for `span.context.service.target.*` and `span.context.destination.service.resource`. Changes may be relevant if your agent has implemented https://github.com/elastic/apm/issues/621.

1. If your agent still derives `destination.service.resource` from span context directly, rather than from `service.target.*` fields, and your agent provides an API for the user to tweak `service.target.*`, then you may want to update.
2. If your agent implements *compressed spans*, then the logic to set `span.name` to `Calls to <...>` has become [slightly more complex.](https://github.com/elastic/apm/pull/674/files#diff-1af43a88144db9f89f119f352647a6d39de152886a4b28deb7b2741b140f7aa0)
3. If your agent implements the OTel Bridge then there is [an update to the compatibility mapping pseudo-code](https://github.com/elastic/apm/pull/674/files#diff-fdb44aa1520898c1f9d91974c73ef587f6a004fb8e7715ef8c01e7018e5e80a5) to set `service.target.*` rather than `destination.service.resource`.

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-dotnet/issues/1849
- [ ] https://github.com/elastic/apm-agent-python/issues/1657
- [ ] https://github.com/elastic/apm-agent-go/issues/1331
- [ ] https://github.com/elastic/apm-agent-php/issues/791