Introduce Mute/None log level

[kriskowal]

YARPC observability middleware allow users to specify a log level for (inbound, outbound) x (success, failure, application error) traffic edges as of https://github.com/yarpc/yarpc-go/pull/1763. Users would like to be able to mute certain edges, such that they can see their own debug messages without seeing YARPC’s.

Currently, the best advice we can provide is to raise the development log level to "info" to suppress the YARPC debug logs, then raise application log messages to "info" during development, requiring the application debug log level to be configurable. This is acceptable but unpalatable.

To this end, would it be wise to introduce a new Mute or None debug level with numeric value -2? If so, I can initiate a change.

[prashantv]

Some related issues: #680 & #713

I think this is a slightly different request -- it's asking for a level that will never be logged, which allows for easily disabling logging via level. If we do this:

• we should probably use a very negative number in case we want to add more levels (like trace) in future. So something like -1000
• I'd prefer None as the name, as the level is used both for logs but also for configuring minimum logger levels. A minimum of Mute seems confusing, whereas a minimum level of None makes more sense
• This only helps if the caller always uses Check for logging, as it's the only way to pass a level parameter.
• We will need to make sure the sampler and other similar components handle this level fine

This makes sense at a high level to me, we should get another opinion too.

[abhinav]

A None level (with a large negative value) sounds reasonable to me.

[jcorbin]

A zap.NoneLevel = -1000 (or so) seems reasonable to me; in addition to sampler (as mentioned by prashant) I suppose we should add guards to all care implementations that zap ships, e.g. zapcore.ioCore.Write, zapcore.multiCore.Write, etc).

[LeviMatus]

If this is still something that is still desired then I'd like to take it up.

[abhinav]

Since adding a new level would be a breaking change, we'd like to avoid doing so in 1.0.

We should defer this to a 2.0, whenever that happens. (https://github.com/uber-go/zap/issues/388)