See #16 (comment)

IDK where it was there to begin with. The [go module spec][1] says:

> The toolchain directive only has an effect when the module is the main
> module and the default toolchain’s version is less than the suggested
> toolchain’s version.

So removing this should not have any impact.

[1]: https://go.dev/ref/mod#go-mod-file-toolchain

This brings us inline with the minimal go version required by
dd-trace-go and also allows using `cmp.Or` which is introduced by a
later commit in this PR.

feat: Add new Emitter API to start and stop metrics

This test was passing entirely by accident in the past because
`/sync/mutex/wait/total:seconds` was typically not zero by the time was
getting executed.

Add proper synchronization to statsdClientMock to prevent data races between
concurrent reads and writes to the call slices. The race occurred when the
emitter goroutine was writing to the slices while the test goroutine was
reading from them.

Changes:
- Add sync.RWMutex to statsdClientMock struct
- Protect all slice writes with write locks in GaugeWithTimestamp, CountWithTimestamp, and DistributionSamples methods
- Add thread-safe getter methods (GaugeCalls, CountCalls, DistributionSampleCalls) that return copies under read lock
- Update all test code to use the new thread-safe getter methods instead of direct slice access

Fixes race condition detected by 'go test -race'

…urns

This fixes flakiness in TestEmitter/should_emit_metrics.

fix test suite flakiness

Change the API of NewEmitter to return an error when there is more than
one instance running already.

Add an AllowMultipleInstances in case we have a need for running
multiple instances at some point for some reason.

This paves the path for reintroducing the old Start() API and is
probably also a good idea in general.

This might help with upgrading some of our internal systems.

* tags: restore SetBaseTags() which is also used internally

* fix: only apply SetBaseTags for the Start() API (#23)

---------

Co-authored-by: Felix Geisendörfer <[email protected]>

…nning

fix: NewEmitter panic if opts is nil and more than one instance is running

Two reasons:

1. Exclude godebug metrics to keep costs under control.
2. Submitting new metrics requires back end changes to get the units
   right.

chore: hard code the list of supported metrics