fluentd forward exporter

I would like to implement fluentd forward exporter for OpenTelemetry C++ SDK for the following modes:

• Forward
• PackedForward
• CompressedPackedForward

Output for the following transports - fluentd inputs :

• in_forward - The in_forward Input plugin listens to a TCP socket to receive the event stream. It also listens to a UDP socket to receive heartbeat messages. See also the protocol section for implementation details.

• in_unix - The in_unix Input plugin enables Fluentd to retrieve records from the Unix Domain Socket. The wire protocol is the same as in_forward, but the transport layer is different.

I started initial work on it. The idea is we can build it as an optional community contribution for OpenTelemetry C++ SDK.

Features

One aspect of exporter that I'd like to cover is semantic convention of the fields. By default the field names for spans, events, attributes, etc. - would follow semantics agreed for the other JSON-alike exporters in OpenTelemetry. But it would be great to add some extensibility. Such as a concept of decorator or transformer that allows to customize the destination attributes. This way the exporter can be used for other vendor destinations, that may need custom field names, positions, and custom tree structure of the actual events carried within the forward protocol.

Start by reviewing the initial work mentioned for the OpenTelemetry C++ SDK and the Fluentd forward protocol requirements. Check how the Forward, PackedForward, and CompressedPackedForward modes map to the in_forward and in_unix transports. Done should include the requested modes and transports, plus a defined approach for semantic conventions and extensible field transformation.