Improve the InfluxDB through-put performance

This changes the current use of the InfluxDB client to instead use a
baked-in client that uses the fasthttp library.

This allows for significantly smaller allocations, the re-use of http
body buffers, and the re-use of the actual bytes of the line-protocol
metric representations.

metric/reader.go

@@ -0,0 +1,155 @@
+package metric
+
+import (
+	"io"
+
+	"github.com/influxdata/telegraf"
+)
+
+type state int
+
+const (
+	_ state = iota
+	// normal state copies whole metrics into the given buffer until we can't
+	// fit the next metric.
+	normal
+	// split state means that we have a metric that we were able to split, so
+	// that we can fit it into multiple metrics (and calls to Read)
+	split
+	// overflow state means that we have a metric that didn't fit into a single
+	// buffer, and needs to be split across multiple calls to Read.
+	overflow
+	// splitOverflow state means that a split metric didn't fit into a single
+	// buffer, and needs to be split across multiple calls to Read.
+	splitOverflow
+	// done means we're done reading metrics, and now always return (0, io.EOF)
+	done
+)
+
+type reader struct {
+	metrics      []telegraf.Metric
+	splitMetrics []telegraf.Metric
+	buf          []byte
+	state        state
+
+	// metric index
+	iM int
+	// split metric index
+	iSM int
+	// buffer index
+	iB int
+}
+
+func NewReader(metrics []telegraf.Metric) io.Reader {
+	return &reader{
+		metrics: metrics,
+		state:   normal,
+	}
+}
+
+func (r *reader) Read(p []byte) (n int, err error) {
+	var i int
+	switch r.state {
+	case done:
+		return 0, io.EOF
+	case normal:
+		for {
+			// this for-loop is the sunny-day scenario, where we are given a
+			// buffer that is large enough to hold at least a single metric.
+			// all of the cases below it are edge-cases.
+			if r.metrics[r.iM].Len() < len(p[i:]) {
+				i += r.metrics[r.iM].SerializeTo(p[i:])
+			} else {
+				break
+			}
+			r.iM++
+			if r.iM == len(r.metrics) {
+				r.state = done
+				return i, io.EOF
+			}
+		}
+
+		// if we haven't written any bytes, check if we can split the current
+		// metric into multiple full metrics at a smaller size.
+		if i == 0 {
+			tmp := r.metrics[r.iM].Split(len(p))
+			if len(tmp) > 1 {
+				r.splitMetrics = tmp
+				r.state = split
+				if r.splitMetrics[0].Len() < len(p) {
+					i += r.splitMetrics[0].SerializeTo(p)
+					r.iSM = 1
+				} else {
+					// splitting didn't quite work, so we'll drop down and
+					// overflow the metric.
+					r.state = normal
+					r.iSM = 0
+				}
+			}
+		}
+
+		// if we haven't written any bytes and we're not at the end of the metrics
+		// slice, then it means we have a single metric that is larger than the
+		// provided buffer.
+		if i == 0 {
+			r.buf = r.metrics[r.iM].Serialize()
+			i += copy(p, r.buf[r.iB:])
+			r.iB += i
+			r.state = overflow
+		}
+
+	case split:
+		if r.splitMetrics[r.iSM].Len() < len(p) {
+			// write the current split metric
+			i += r.splitMetrics[r.iSM].SerializeTo(p)
+			r.iSM++
+			if r.iSM >= len(r.splitMetrics) {
+				// done writing the current split metrics
+				r.iSM = 0
+				r.iM++
+				if r.iM == len(r.metrics) {
+					r.state = done
+					return i, io.EOF
+				}
+				r.state = normal
+			}
+		} else {
+			// This would only happen if we split the metric, and then a
+			// subsequent buffer was smaller than the initial one given,
+			// so that our split metric no longer fits.
+			r.buf = r.splitMetrics[r.iSM].Serialize()
+			i += copy(p, r.buf[r.iB:])
+			r.iB += i
+			r.state = splitOverflow
+		}
+
+	case splitOverflow:
+		i = copy(p, r.buf[r.iB:])
+		r.iB += i
+		if r.iB >= len(r.buf) {
+			r.iB = 0
+			r.iSM++
+			if r.iSM == len(r.splitMetrics) {
+				r.iM++
+				r.state = normal
+			} else {
+				r.state = split
+			}
+		}
+
+	case overflow:
+		i = copy(p, r.buf[r.iB:])
+		r.iB += i
+		if r.iB >= len(r.buf) {
+			r.iB = 0
+			r.iM++
+			if r.iM == len(r.metrics) {
+				r.state = done
+				return i, io.EOF
+			}
+			r.state = normal
+		}
+	}
+
+	return i, nil
+}