log: support context-logging

[protolambda]

Description

Changes:

• Support the context-logging interface of slog.Logger
  • Additionally includes TraceContext.
  • Intentionally does not include CritContext (crits are discouraged, and we would never want to filter from output based on context)
• Add WriteCtx to complement the existing Write method
• Allow it to set a default context on the logger, to use in case none is specified
• Support slog.Attr in regular log calls
• Fix test-logger to actually handle Write
• Fix test-logger to handle Crit more nicely

Tests

• Test context is preserved in logging, so that the handler can use it
• Test that testlog Crit does work nicely
• Test that slog.Attr can be used in regular log calls
• Vmodule test still works (call-site detection)

Additional context

This helps us remove a big logging hack in our monorepo testing, so that we can do context-logging and test tracing properly.

We can also attach the devtest test-scope context to our loggers, so logs can be attributed to scopes with context.

And we can attach attributes like service-ID to package-level loggers, so we can make the log-handler easily filter down to only logs with relevant context.

Metadata

[protolambda]

CI is broken due to upstream EELS release issue. Fixing in #612

[sebastianst]

Would be nice to open this PR upstream! Seems like a general improvement that's compatible with the existing API.