Automatically set GOMAXPROCS

[cmacknz]

Describe the enhancement:
Automatically set GOMAXPROCS when beats are run in containers with a CFS quota set to minimize CPU throttling.

See elastic/apm-server#7967 and elastic/apm-server#8278 for more context. https://github.com/uber-go/automaxprocs can be used to easily add this feature.

Describe a specific use case for the enhancement or feature:
Setting GOMAXPROCS based on the configured CPU quota makes performance more predictable in containerized environments. Kubernetes deployments are particularly likely to experience problems from unoptimized GOMAXPROCS configurations.

Clone of elastic/beats#31920 for the shipper.