Automatically set GOMAXPROCS based on cgroup quotas

[cirocosta]

challenge

Go will leverage the number of online cpus on the machine when defining how many threads to spawn to build the pool of schedulable threads for the goroutines.

While that's very desirable when having the entire machine for yourself, that is not the most optimal when running in containers having CPU limitted - e.g., see some benchmarks: uber-go/automaxprocs#12

would be better

To tune that, one can manually set GOMAXPROCS and keep that in sync with the quotas assigned to the cgroups where Concourse runs, but that's easy to forget if not automated.

It'd be nice if Concourse by itself, knowing that it runs in a cgroup, were able to set that config.

proposal

we could leverage https://github.com/uber-go/automaxprocs to automatically have that value set.

before doing so, it might be worth experimenting with having GOMAXPROCS with a synthetic concourse deployment (probably running the algorithm stress environment?)