Add support for metadata in Fluentd event structure

### Is your feature request related to a problem? Please describe.

Currently, Fluentd handles the following events structure:

```
[TAG, TIMESTAMP, {RECORD}]
```

We wanted to change to handle metadata in Fluentd event structure:

```
[TAG, [TIMESTAMP, {METADATA}], {RECORD}]
```

Note that, in the above structure, timestamp is already one of the metadata structure and other metadata should be handled in msgpack map that is represented Hash in Ruby.

### Describe the solution you'd like

Handle metadata requirements in Fluentd.

### Describe alternatives you've considered

The minimum solution is just handled as normal Forwarded events with metadata from fluent-bit:

From fluent-bit, events metadata should be sent as:

```
[TAG, TIMESTAMP, {RECORD}, {metadata key/val in OPTION}]
```

And metadata part should be stored in a special key. However, supporting metadata in the second part of the event should be simpler for long term perspective.

### Additional context

We'd discussed this RFC in fluent-bit issues:
https://github.com/fluent/fluent-bit/issues/6666

UPDATE:

Also, Fluent Bit with out_forward just uses `options` to represent their metadata. We don't expect any breakage for sending logs type of Fluent Bit events from Fluent Bit to Fluentd.