[WIP] Add option to increase the level of a logger

We've had a few requests on how to change the level of a logger. With
the current APIs, it's not possible to reduce the log level (e.g., go
from Info to Debug), but it is possible to increase the level using
`WrapCore` with a custom filtering core.

Since it's a common request, I think we should add an option to make
this easy to use.

I want to make sure "increase" is part of the API to avoid confusion on
whether it can reduce the log level.

Fixes #774.

logger_test.go

@@ -392,6 +392,21 @@ func TestLoggerReplaceCore(t *testing.T) {
 	})
 }
 
+func TestLoggerIncreaseLevel(t *testing.T) {
+	withLogger(t, DebugLevel, opts(IncreaseLevel(WarnLevel)), func(logger *Logger, logs *observer.ObservedLogs) {
+		logger.Info("logger.Info")
+		logger.Warn("logger.Warn")
+		logger.Error("logger.Error")
+		require.Equal(t, 2, logs.Len(), "Expected no-op core to write no logs.")
+		assert.Equal(
+			t,
+			logs.AllUntimed()[0].Entry.Message,
+			"logger.Warn",
+			"Expected first logged message to be warn level message",
+		)
+	})
+}
+
 func TestLoggerHooks(t *testing.T) {
 	hook, seen := makeCountingHook()
 	withLogger(t, DebugLevel, opts(Hooks(hook)), func(logger *Logger, logs *observer.ObservedLogs) {

options.go

@@ -107,3 +107,10 @@ func AddStacktrace(lvl zapcore.LevelEnabler) Option {
 		log.addStack = lvl
 	})
 }
+
+// IncreaseLevel increase the level of the logger.
+func IncreaseLevel(lvl zapcore.LevelEnabler) Option {
+	return WrapCore(func(c zapcore.Core) zapcore.Core {
+		return zapcore.NewLevelCore(c, lvl)
+	})
+}

zapcore/level_filter.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2019 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package zapcore
+
+type levelFilterCore struct {
+	Core
+	level LevelEnabler
+}
+
+// NewLevelCore creates a core that can be used to increase the level of an existing
+// Core. It cannot be used to decrease the logging level, as it acts as a filter
+// before calling the underlying core.
+func NewLevelCore(core Core, level LevelEnabler) Core {
+	return &levelFilterCore{core, level}
+}
+
+func (c *levelFilterCore) Enabled(lvl Level) bool {
+	return c.level.Enabled(lvl)
+}
+
+func (c *levelFilterCore) Check(ent Entry, ce *CheckedEntry) *CheckedEntry {
+	if !c.Enabled(ent.Level) {
+		return ce
+	}
+
+	return c.Core.Check(ent, ce)
+}