Add tunable parameters for Nexus config

[bnaecker]

• Add the [tunables] section to the nexus config
• Includes a key max_vpc_ipv4_subnet_prefix. This takes the place of
  the compile-time constant
  omicron_nexus::defaults::MAX_VPC_IPV4_SUBNET_PREFIX. The proximal
  reason for this is to allow a small subnet when testing, to keep the
  subnet allocation integration test short.

[bnaecker]

For the record:

On main:

bnaecker@shale : ~/omicron $ time cargo t -- integration_tests::subnet_allocation
    Finished test [unoptimized + debuginfo] target(s) in 0.37s
     Running unittests src/lib.rs (target/debug/deps/authz_macros-174b819b1c918d21)
...

real	1m23.797s
user	1m22.112s
sys	0m10.396s

On this branch:

bnaecker@shale : ~/omicron $ time cargo t -- integration_tests::subnet_allocation
    Finished test [unoptimized + debuginfo] target(s) in 0.37s
     Running unittests src/lib.rs (target/debug/deps/authz_macros-174b819b1c918d21)
...
real	0m26.130s
user	0m17.913s
sys	0m6.578s
bnaecker@shale : ~/omicron $

[david-crespo]

Remember there are a couple of other configs to update.

https://github.com/oxidecomputer/omicron/blob/main/nexus/examples/config.toml
https://github.com/oxidecomputer/omicron/blob/main/nexus/examples/config-file.toml

I wonder if it's even worth keeping these around, though. Is anyone using them? If so maybe we should have a test to make sure they're kept up to date (just musing, that would be a separate PR).

Also: there's some other stuff in the config that arguably could belong under tunables — is that worth moving around? Meh.

omicron/nexus/examples/config.toml

Line 33 in f02f5a3

request_body_max_bytes = 1048576

[bnaecker]

Yeah, moving the maximum request body might be a good idea. I specifically didn't add the tunables to the other configuration files, though, since I want those to use the default.

I'm not sure anything uses the config-file.toml, but we're definitely still using config.toml.

[david-crespo]

Ah, missed the #[serde(default)] bit. That wasn't being used by anything last time I had occasion to look at config.rs. 👍

[davepacheco]

Thanks for doing this! That's a nice improvement.

Re: the config files: I definitely use config.toml. I think it's nice to have an example of a file-based logger, but that could also be commented-out in config.toml instead. For the same reason, I'd kind of like to see the tunables in here. To me, the example config file is documentation + example as much as something for people to use directly.

I also think it'd be nice to have a test that at least parses all the config files. (Agreed: not part of this PR.)