docs(codecs): Document the output type of each decoder

[matthijskooijman]

Summary

This documents the output type (metric/logs/traces) generated by each decoder, based out the output_type methods in the decoder. Previously, this was only documented for the decoders that generated multiple types, but not for the ones that only generate logs or metrics.

How did you test this PR?

Used make generate-component-doc to regenerate cue files (which I presume is the right thing to do for such a change, but it does not seem to be documented in CONTRIBUTING.md or other places I looked). Did not see how to generate actual HTML to see if the result looks ok.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

[matthijskooijman]

One more thing, that I noticed while creating this PR: The OTLP codec is documented to support all three output types, and also seems to have code for all three, but seems to only advertise two of them in the code:

vector/lib/codecs/src/decoding/format/otlp.rs

Lines 69 to 72 in c5f8995

	/// Return the type of event build by this deserializer.
	pub fn output_type(&self) -> DataType {
	DataType::Log | DataType::Trace
	}

I have not idea what this means, but it seems inconsistent and maybe a bug?

[pront]

One more thing, that I noticed while creating this PR: The OTLP codec is documented to support all three output types, and also seems to have code for all three, but seems to only advertise two of them in the code:

vector/lib/codecs/src/decoding/format/otlp.rs

Lines 69 to 72 in c5f8995

	/// Return the type of event build by this deserializer.
	pub fn output_type(&self) -> DataType {
	DataType::Log | DataType::Trace
	}

I have not idea what this means, but it seems inconsistent and maybe a bug?

This is a documented limitation:

https://vector.dev/docs/reference/configuration/sources/opentelemetry/#otlp-decoding-usage

However, OTLP formatted metrics cannot be converted to Vector’s metrics format. As a workaround, the OTLP metrics are converted to Vector log events while preserving the OTLP format. This prohibits the use of metric transforms like aggregate but it enables easy shipping to OTEL collectors.

More context here in the comments here: #17310 (comment)

[matthijskooijman]

Ok, thanks. I'm not sure that I understand completely, but that is fine :-)