Agent/beats gRPC over domain sockets/named pipes

[aleksmaus]

Describe the enhancement:

Change the Agent/Beats gRPC communication to utilize domain sockets/named pipes.

This is to follow up on the initial feature request: Agent/beats grpc comms over domain socket/named pipe

Done:

The initial implementation with Agent/beats grpc comms over domain socket/named pipe PR was scaled back due to lack of support for gRPC over domain sockets/named pipes by the underlying library that Endpoint uses for comms. The scope of this change was limited to switching to domain socket/named pipe for connection information discovery only, for 8.15 release.

• The elastic-agent-client library was updated in order to support comms over domain socket/named piped: https://github.com/elastic/elastic-agent-client/pull/91

• The Endpoint spec was enhanced with csocket configuration that defines the domain socket file name relative to the Agent install directory on non-Windows OS or the named pipe name on Windows.

• The Endpoint was modified to connect to the specified domain socket/named pipe for the connection information discovery.

• The code for domain sockets/named pipes gRPC comms was disabled in https://github.com/elastic/elastic-agent/pull/4249

TODO:

• Endpoint to implement gRPC comms over domain sockets/named pipes support. There are plans to replace/rewrite the existing gRPC comms.
• Enable the Agent code for domain sockets/named piped gPRC, that was disabled here https://github.com/elastic/elastic-agent/pull/4249/commits/0b8ca7b56e118e6f5a1a93846cf3f5308fd183a6

Describe a specific use case for the enhancement or feature:

There were few SDH related to the fact that the current GRPC comms are over IP socket and ask for ability to use domain socket/named pipe instead.

What is the definition of done?

Agent can communicate with Beats and Security Endpoint over domain sockets/named pipes.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)