v2 metrics

[lavoiesl]

Significant refactor of the statsd package.

Design principles / Motivation

Backends' features to add prefix, tags, and context extraction should be composable, allowing users to pick and choose what they want from the library.

New features

1. Is it now possible to set a context-specific backend by using ContextWithBackend. If it is not specified, the default backend is still honoured.

Breaking changes

1. NewLogBackend is replaced with NewLogrusBackend to better reflect its implementation
   2.NewLogBackend and NewDatadogBackend no longer have built-in tags extraction from Context, prefix, and default tags. Instead, users should call BackendWithDefaultWrappers or wrap them manually.
2. The signature of the forwarding handler has changed to use a Metric struct to simplify implementations
3. A MockBackend has replaced the mocks.Backend
4. The statsd package was renamed to metrics, to better reflect that it is not statsd specific.
5. The Timer metric now uses the Timing function and the redundant Timing metric has been removed.
6. The Backend signature has changed to receive Tags maps instead of string slices. String slices are now only created when calling the underlying statsd client.

Minor changes

1. Move goose's metric declarations to individual packages, to avoid cross-package dependencies.
2. logError now reports an error instead of a warning when failing to emit a metric.
3. Tags now has helper methods to merge with other tags and generate a slice of strings.

[wperron]

Are we keeping this?

[lavoiesl]

Yes, Timing is used by the Timer metric.