feat(metrics): add basic metrics type support

[viglia]

this PR aims at adding metrics type definition and the related serialization logic.

This does not include high-level APIs that will be used by the users (with proper aggregation/bucketing).

Example of how to send metrics through the Transport struct:

package main

import (
	"time"

	"strings"

	"github.com/getsentry/sentry-go"
	"github.com/google/uuid"
)

func main() {
	id := strings.Replace(uuid.New().String(), "-", "", -1)
	e := sentry.NewEvent()
	e.EventID = sentry.EventID(id)

	e.Type = "statsd"
	t := time.Now().Unix()
	release := "1.0"
	env := "dev"
	e.Metrics = []sentry.Metric{
		sentry.NewCounterMetric("test-go-metric", sentry.Second(), map[string]string{"transaction": "go-metrics", "release": release, "env": env, "foo": "bar"}, t, 1.0),
		sentry.NewCounterMetric("test-go-metric", sentry.Second(), map[string]string{"transaction": "go-metrics", "release": release, "env": env, "route": " /foo"}, t, 1.0),
	}
	tr := sentry.NewHTTPSyncTransport()
	tr.Configure(sentry.ClientOptions{
		Dsn:   "<YOUR_DSN_HERE>",
		Debug: true,
	})

	tr.SendEvent(e)

}

[codecov]

Codecov Report

Attention: Patch coverage is 58.64979% with 98 lines in your changes are missing coverage. Please review.

Project coverage is 81.74%. Comparing base (84427d8) to head (1405211).
Report is 1 commits behind head on master.

Files	Patch %	Lines
metrics.go	55.67%	86 Missing ⚠️
transport.go	72.09%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
- Coverage   83.15%   81.74%   -1.41%     
==========================================
  Files          49       50       +1     
  Lines        4190     4427     +237     
==========================================
+ Hits         3484     3619     +135     
- Misses        565      659      +94     
- Partials      141      149       +8     

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[cleptric]

Please add some usage examples to the PR description. Did you already send some metrics to Sentry?

[aldy505]

To be honest, this metrics implementation would be so easy if we did #643 first.

[aldy505]

Any updates for this?