add bugsnag logrus hook

Signed-off-by: Matt Tescher <matthew.tescher@docker.com>

vendor/github.com/Shopify/logrus-bugsnag/LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2016 Shopify
+
+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.

vendor/github.com/Shopify/logrus-bugsnag/README.md

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+## logrus-bugsnag
+
+[![Build Status](https://travis-ci.org/Shopify/logrus-bugsnag.svg)](https://travis-ci.org/Shopify/logrus-bugsnag)
+
+logrus-bugsnag is a hook that allows [Logrus](https://github.com/sirupsen/logrus) to interface with [Bugsnag](https://bugsnag.com).
+
+#### Usage
+
+```go
+import (
+  log "github.com/sirupsen/logrus"
+  "github.com/Shopify/logrus-bugsnag"
+  bugsnag "github.com/bugsnag/bugsnag-go"
+)
+
+func init() {
+  bugsnag.Configure(bugsnag.Configuration{
+    APIKey: apiKey,
+  })
+  hook, err := logrus_bugsnag.NewBugsnagHook()
+  logrus.StandardLogger().Hooks.Add(hook)
+}
+```
+

vendor/github.com/Shopify/logrus-bugsnag/bugsnag.go

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+package logrus_bugsnag
+
+import (
+	"errors"
+
+	"github.com/sirupsen/logrus"
+	"github.com/bugsnag/bugsnag-go"
+	bugsnag_errors "github.com/bugsnag/bugsnag-go/errors"
+)
+
+type bugsnagHook struct{}
+
+// ErrBugsnagUnconfigured is returned if NewBugsnagHook is called before
+// bugsnag.Configure. Bugsnag must be configured before the hook.
+var ErrBugsnagUnconfigured = errors.New("bugsnag must be configured before installing this logrus hook")
+
+// ErrBugsnagSendFailed indicates that the hook failed to submit an error to
+// bugsnag. The error was successfully generated, but `bugsnag.Notify()`
+// failed.
+type ErrBugsnagSendFailed struct {
+	err error
+}
+
+func (e ErrBugsnagSendFailed) Error() string {
+	return "failed to send error to Bugsnag: " + e.err.Error()
+}
+
+// NewBugsnagHook initializes a logrus hook which sends exceptions to an
+// exception-tracking service compatible with the Bugsnag API. Before using
+// this hook, you must call bugsnag.Configure(). The returned object should be
+// registered with a log via `AddHook()`
+//
+// Entries that trigger an Error, Fatal or Panic should now include an "error"
+// field to send to Bugsnag.
+func NewBugsnagHook() (*bugsnagHook, error) {
+	if bugsnag.Config.APIKey == "" {
+		return nil, ErrBugsnagUnconfigured
+	}
+	return &bugsnagHook{}, nil
+}
+
+// skipStackFrames skips logrus stack frames before logging to Bugsnag.
+const skipStackFrames = 4
+
+// Fire forwards an error to Bugsnag. Given a logrus.Entry, it extracts the
+// "error" field (or the Message if the error isn't present) and sends it off.
+func (hook *bugsnagHook) Fire(entry *logrus.Entry) error {
+	var notifyErr error
+	err, ok := entry.Data["error"].(error)
+	if ok {
+		notifyErr = err
+	} else {
+		notifyErr = errors.New(entry.Message)
+	}
+
+	metadata := bugsnag.MetaData{}
+	metadata["metadata"] = make(map[string]interface{})
+	for key, val := range entry.Data {
+		if key != "error" {
+			metadata["metadata"][key] = val
+		}
+	}
+
+	errWithStack := bugsnag_errors.New(notifyErr, skipStackFrames)
+	bugsnagErr := bugsnag.Notify(errWithStack, metadata)
+	if bugsnagErr != nil {
+		return ErrBugsnagSendFailed{bugsnagErr}
+	}
+
+	return nil
+}
+
+// Levels enumerates the log levels on which the error should be forwarded to
+// bugsnag: everything at or above the "Error" level.
+func (hook *bugsnagHook) Levels() []logrus.Level {
+	return []logrus.Level{
+		logrus.ErrorLevel,
+		logrus.FatalLevel,
+		logrus.PanicLevel,
+	}
+}