Implement telegraf's own full metric type

main reasons behind this:
- make adding/removing tags cheap
- make adding/removing fields cheap
- make parsing cheaper
- make parse -> decorate -> write out bytes metric flow much faster

Refactor serializer to use byte buffer

metric/metric_test.go

@@ -0,0 +1,107 @@
+package metric
+
+import (
+	"fmt"
+	"math"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewMetric(t *testing.T) {
+	now := time.Now()
+
+	tags := map[string]string{
+		"host":       "localhost",
+		"datacenter": "us-east-1",
+	}
+	fields := map[string]interface{}{
+		"usage_idle": float64(99),
+		"usage_busy": float64(1),
+	}
+	m, err := New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+
+	assert.Equal(t, Untyped, m.Type())
+	assert.Equal(t, tags, m.Tags())
+	assert.Equal(t, fields, m.Fields())
+	assert.Equal(t, "cpu", m.Name())
+	assert.Equal(t, now, m.Time())
+	assert.Equal(t, now.UnixNano(), m.UnixNano())
+}
+
+func TestNewGaugeMetric(t *testing.T) {
+	now := time.Now()
+
+	tags := map[string]string{
+		"host":       "localhost",
+		"datacenter": "us-east-1",
+	}
+	fields := map[string]interface{}{
+		"usage_idle": float64(99),
+		"usage_busy": float64(1),
+	}
+	m, err := New("cpu", tags, fields, now, Gauge)
+	assert.NoError(t, err)
+
+	assert.Equal(t, Gauge, m.Type())
+	assert.Equal(t, tags, m.Tags())
+	assert.Equal(t, fields, m.Fields())
+	assert.Equal(t, "cpu", m.Name())
+	assert.Equal(t, now, m.Time())
+	assert.Equal(t, now.UnixNano(), m.UnixNano())
+}
+
+func TestNewCounterMetric(t *testing.T) {
+	now := time.Now()
+
+	tags := map[string]string{
+		"host":       "localhost",
+		"datacenter": "us-east-1",
+	}
+	fields := map[string]interface{}{
+		"usage_idle": float64(99),
+		"usage_busy": float64(1),
+	}
+	m, err := New("cpu", tags, fields, now, Counter)
+	assert.NoError(t, err)
+
+	assert.Equal(t, Counter, m.Type())
+	assert.Equal(t, tags, m.Tags())
+	assert.Equal(t, fields, m.Fields())
+	assert.Equal(t, "cpu", m.Name())
+	assert.Equal(t, now, m.Time())
+	assert.Equal(t, now.UnixNano(), m.UnixNano())
+}
+
+func TestNewMetricString(t *testing.T) {
+	now := time.Now()
+
+	tags := map[string]string{
+		"host": "localhost",
+	}
+	fields := map[string]interface{}{
+		"usage_idle": float64(99),
+	}
+	m, err := New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+
+	lineProto := fmt.Sprintf("cpu,host=localhost usage_idle=99 %d\n",
+		now.UnixNano())
+	assert.Equal(t, lineProto, m.String())
+}
+
+func TestNewMetricFailNaN(t *testing.T) {
+	now := time.Now()
+
+	tags := map[string]string{
+		"host": "localhost",
+	}
+	fields := map[string]interface{}{
+		"usage_idle": math.NaN(),
+	}
+
+	_, err := New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+}