Add logger to bbolt

[ahrtr]

Currently bbolt only returns error to caller. It should support printing log message as well.

We can follow the same way as raft does,

• Define a Logger interface, and the caller can pass into any implementation they like.
• bbolt implements a default implementation if users do not pass in any logger implementation.
• Add log messages. e.g. all error cases should have an error message.

[serathius]

I'm a little out of touch with golang slog, but it's better to take an existing logging interface then rollout your own. For picking:

• https://github.com/go-logr/logr - interface used by K8s.
• https://pkg.go.dev/golang.org/x/exp/slog - experimental interface by golang team.

[ahrtr]

Proposal 1:

Just as I mentioned in #509 (comment), we follow the same way as what we does for etcd and raft.

• Define an simple Logger interface in bbolt.
• Make very minor change in etcd, so that the existing Logger implementation for raft can be reused for bbolt as well.

The minor concern is there is duplicated Logger interface definition in both bbolt and raft. It should be extracted into a separate common repo.

The other concern is that it isn't structured log. It isn't a big problem for etcd, because the log will be encapsulated into JSON format by etcd.

Proposal 2:

1. Introduce https://github.com/go-logr/logr into bbolt;
2. Use the fnlogger as the default implementation if users do not provide a LogSink implementation. See example NewStdoutLogger;
3. Add an simeple & thin adapter for uber-go/zap in etcd to implement the LogSink interface.

The good side is it's structured log.

The concern of this proposal is logr only has Error and Info, and Verbosity can be used to control the level of info log. It doesn't match uber-go/zap's level definition: debug, info, warn, error, panic and fatal. It means we need to add additional conversion between the two kinds of log Verbosity/level in etcd. For example, we can regard all level > 0 as debug log.

etcd already has a flag --log-level (see below) for uber-go/zap, we definitely don't want to add one more flag something like --log-verbose for go-logr/logr,

  --log-level 'info'
    Configures log level. Only supports debug, info, warn, error, panic, or fatal.

[cenkalti]

A library should not output any logs by default. That is application's responsibility. We can provide a logger field that users can override with their own implementation.

[ahrtr]

A library should not output any logs by default.

Basically agree with this. Otherwise the bbolt CLI may output some unexpected log messages as well.

We can set the default logger to discardLogger, so that the logger always points to a valid instance, and accordingly we don't need to check nil pointer something like if lg != nil {...}.

[cenkalti]

Proposal 3:

Use https://pkg.go.dev/golang.org/x/exp/slog as it is more likely to get adopted by the community and become a standard. The user can chose whatever handler he want want with it.

[ahrtr]

We do not need fancy features, we just need a simple logger interface, so that users can provide whatever logger implementation they want. Note that etcd already uses go.uber.org/zap, and proposal 1 has already been proved that it works well with uber.