out_prometheus_remote_write: concatenate cmetrics buffers
out_prometheus_remote_write: concatenate cmetrics buffers

This patch generate a concatenation of multiple cmetrics buffers when received on the
flush callback. Serializing the payloads works just fine (tested with Grafana cloud service).

In addition I've added some debug callbacks for testing, e.g:

    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] cmetrics msgpack size: 289736
    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] cmetric_id=0 decoded 0-72434 payload_size=41486
    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] cmetric_id=1 decoded 72434-144868 payload_size=41486
    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] cmetric_id=2 decoded 144868-217302 payload_size=41486
    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] cmetric_id=3 decoded 217302-289736 payload_size=41486
    [2021/07/25 20:35:31] [debug] [output:prometheus_remote_write:...0] final payload size: 165944
    [2021/07/25 20:35:32] [ info] [output:prometheus_remote_write:...0] prometheus-us-central1.grafana.net:443, HTTP status=200
    [2021/07/25 20:35:32] [debug] [output:prometheus_remote_write:...0] http_post result FLB_OK

Signed-off-by: Eduardo Silva <b6525c140147034c280e3cd2f39161f32f3f4f62@gmail.com>

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