We'll probably still have some way to go, but this PR attempts to make consumers of this package less coupled to logrus.

These are still aliases, but allowing consumers to use the types defined by this package allows us to switch definitions at some point.

log: remove testify dependency

Testify was only used for a basic assertion. Remove the dependency,
in preparation of (potentially) moving this package to a separate
module.

log: SetFormat: include returns in switch

log: WithLogger: remove redundant intermediate var

log: group "enum" consts and touch-up docs

Also updated the level descriptions with their documentation from
logrus.

log: add all log-levels that are accepted

While other log-levels are not currently used in containerd itself,
they can be returned by GetLevel(), and are accepted (no error) by
SetLevel(). We should either accept those values, or produce an
error (in SetLevel()), but given that there's other ways to set the
log-level, we should probably acknowledge that this package is a transitional
package, and still closely tied to logrus (for the time being).

log: define G() as a function instead of a variable

The G variable is exported, and not expected to be overwritten
externally. Defining it as a function also documents it as a function
on https://pkg.go.dev, instead of a variable; https://pkg.go.dev/github.com/containerd/[email protected]/log#pkg-variables

Note that (while the godoc suggests otherwise) I made GetLogger an alias
for G, as G is the most commonly used function (not the other way round),
although I don't think there's a performance gain in doing so.

log: define OutputFormat type

Strong-type the format.

log: add log.Entry type

Don't return logrus types from exported functions.

log: make Fields type a generic map[string]any

Decouple it from logrus, but with the same type.

log: add package documentation and summary of package's purpose

Add a package doc to (try to) describe the purpose of this package, and
to describe the purpose (and expectations) of aliases provided by the
package.

Package log provides types and functions related to logging, passing
loggers through a context, and attaching context to the logger.

Transitional types

This package contains various types that are aliases for types in [logrus].
These aliases are intended for transitioning away from hard-coding logrus
as logging implementation. Consumers of this package are encouraged to use
the type-aliases from this package instead of directly using their logrus
equivalent.

The intent is to replace these aliases with locally defined types and
interfaces once all consumers are no longer directly importing logrus
types.

IMPORTANT: due to the transitional purpose of this package, it is not
guaranteed for the full logrus API to be provided in the future. As
outlined, these aliases are provided as a step to transition away from
a specific implementation which, as a result, exposes the full logrus API.
While no decisions have been made on the ultimate design and interface
provided by this package, we do not expect carrying "less common" features.

log: swap logrus functions with their equivalent on default logger

logrus.SetLevel(), logrus.GetLevel() and logrus.SetFormatter()
are all convenience functions to configure logrus' standardlogger, which is the
logger to which we hold a reference in the Entry configured on log.L.

This patch:

• swaps calls to logrus.SetLevel, logrus.GetLevel and logrus.SetFormatter
  for their equivalents on log.L. This makes it clearer what SetLevel does,
  and makes sure that we set the log-level of the logger / entry we define in
  our package (even if that would be swapped with a different instance).
• removes the use of logrus.NewEntry with directly constructing a Entry,
  using the local Entry alias (anticipating we can swap that type in future).

With this PR: