Remove global Logger

[rhcarvalho]

A global Logger in a library like sentry-go poses some problems.

• Limited ability to filter logs by Hub, Client or other component. Either all log messages are on or all off. Things can get unnecessary chatty.
• Imposed dependency on log package from Go's standard library, where user may want to use a different logging library (unfortunately, the stdlib has its shortcomings in this case). People should be free to use whatever logging framework they want (and possibly already use in their main package), be it log, go.uber.org/zap, github.com/golang/glog, k8s.io/klog, github.com/sirupsen/logrus, ..., the list is really long ⇒ awesome-go#logging.
• A shared global Logger makes it harder to test.
• Innocent-looking constructors have global side-effects (#148).

Random thoughts

Our logging usage is small. There are <30 calls to Logger.Print*. Possibly all messages would fit a "INFO" level (but we can leave that to package main, i.e. users, to decide what logging facility to use). We may or may not embrace some form of structured logging.

One way to allow users of sentry-go to plug-in their own loggers is to make the SDK depend on a simple interface that can be easily implemented by actual loggers.

At the moment we log from client and transport, hub and scope, and integrations (perhaps oddly including the fasthttp integration). We may need to change some of those log calls.

A call to sentry.Init might take a logger and share it with components it initializes: the default integrations, client, transport, hub, scope, etc.

We may provide a simple way to write to os.Stderr (see #154) for easy debugging.

Resources

• https://dave.cheney.net/2017/01/23/the-package-level-logger-anti-pattern
• Go logging-related discussions in other projects like Kubernetes, Prometheus, coredns, ...
• https://github.com/go-logr/logr

• Posts in golang-nuts. https://groups.google.com/forum/#!topic/golang-dev/F3l9Iz1JX4g https://groups.google.com/d/msg/golang-dev/F3l9Iz1JX4g/rLIZvGvZBgAJ ⇒ https://golang.org/pkg/net/http/httptrace/ =>

  package sentry
  
  type Debug struct {
  EventDropped func(reason string, event *Event)
  MarshalError func(err error, event *Event)
  ...
  }

[github-actions[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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