Organize chronicle exporter by http and grpc

[djaglowski]

This PR contains many changes so it may be wise to chunk it out into smaller changes. However, I'm opening it as a draft in order to facilitate feedback on the overall direction.

Changes included:

• Separate "http exporter" and "grpc exporter".
• Host metrics collection code is more clearly limited to the grpc exporter.
• Separate marshaling code & deletes mocking of marshaler. We shouldn't be mocking things that are end-to-end testable in a local environment.
• Implement mock servers to maximize code coverage. Specifically, this allows us to exercise most of the client code.
• It's still necessary to mock a few functions, basically limited to building endpoints and bypassing authentication.

Future work:

• No changes to configuration for the sake of backwards compatibility. However, it's quite clear that having all configuration at the same level in a single struct makes it difficult to logically enforce the rules. We can consider whether to migrate this later.
• This does not yet include tests for splitting payloads, but we should be able to add them more easily now, since we are not mocking the entire clients.

[djaglowski]

Several smaller PRs have been peeled off and merged. What's left is a large diff but is almost entirely just moving the marshaler into an internal package and separating the functionality into http and grpc variants. The benefit of doing this is that it is now tested according to its exported surface area only, but without exposing the exported surface area to other packages.