observe: warn on unknown field while JSON decoding

[kaworu]

Before this patch, when parsing JSON flows and encountering an unknown field, the CLI would log a warning and skip the flow.

Unknown fields is not uncommon however, e.g. when using the Hubble CLI against a more recent Hubble server version.

This patch

• emit the flow if successfully parsed while ignoring unknown fields,
• improve the warning message by hinting at upgrading the Hubble CLI,
• make it so the warning message is logged only once.

[kaworu]

Opening as draft as it will conflict with #958 and also unsure whether logging a warning message is the way to go.

[michi-covalent]

time to ship

[glibsm]

how is this going to be set?

[glibsm]

is there nothing in the error that we can check to make sure it was unknown field, rather than attempting to parse twice?

[kaworu]

Tracking the error from protobuf it's from

hubble/vendor/google.golang.org/protobuf/encoding/protojson/decode.go

Line 197 in bb32734

return d.newError(tok.Pos(), "unknown field %v", tok.RawString())

and

hubble/vendor/google.golang.org/protobuf/encoding/protojson/decode.go

Lines 96 to 101 in bb32734

	// newError returns an error object with position info.
	func (d decoder) newError(pos int, f string, x ...interface{}) error {
	line, column := d.Position(pos)
	head := fmt.Sprintf("(line %d:%d): ", line, column)
	return errors.New(head+f, x...)
	}

So I see no way to errors.Is as it isn't wrapped. Matching on the error message doesn't feel right either.

[chancez]

This only seems to update the io reader observer, what about the gRPC observer client?

[kaworu]

This only seems to update the io reader observer, what about the gRPC observer client?

Yes good question, so functionally the gRPC client already ignore unknown fields. If we think the warning would be a good idea too for gRPC let's do it.

[chancez]

@kaworu Ahhh, I see. Perhaps you could update the PR title/commit message to reflect this is only when ingesting flows from a file/stdin.

[chancez]

Code changes LGTM.