[Fleet integration] Zero-configuration support for APM agents

[felixbarny]

To decrease initial setup costs per application, APM agents should not require any configuration to be set in order to start sending data to a local APM Server. After the initial onboarding, users can still customize settings if the defaults aren't fitting but that should not stand in the way of the initial onboarding process.

This issue is in the context of the APM Agent Fleet integration. Zero-config is especially important for agents that have some kind of auto-attachment capabilities, such as the agents for .NET, Java, Node.js, PHP, and potentially Python. But even agents such as Go and Ruby will benefit from users not having to worry about configuration just to get started. It would condense their installation instructions down to just adding a dependency on the agent (Ruby) or adding a few snippets to the code (Python).

Agents should be able to auto-detect the APM Server endpoint by just defaulting to localhost:8200. Having the APM Server installed locally will be the default experience with Fleet and enables tail-based sampling to scale horizontally.
Furthermore, agents should detect a sensible default service.name as well as the service.version, if possible.

Relates to #381

Status

Summary:
SPEC issue:

Agent	Zero-configuration support
Java
dot-net
Go
PHP
Ruby
NodeJS
Python

[basepi]

service.name will be the biggest hurdle for the python agent for zero config. The best idea I have so far is to inspect stack frames and try to divine the name that way, but it's probably going to be tricky and brittle. Figured it was worth managing expectations.