kv,sql: test propagation of trace spans on error

[tbg]

The KV layer does (or at least should) propagate trace recordings on errors. We the *BatchResponse as a carrier, which also carries any *Error.

cockroach/pkg/server/node.go

Lines 980 to 982 in 81c9c91

	if rec := tracing.GetRecording(grpcSpan); rec != nil {
	br.CollectedSpans = append(br.CollectedSpans, rec...)
	}

The SQL layer uses RemoteProducerMetadata (or ProducerMetadata? I'm not quite sure):

cockroach/pkg/sql/execinfrapb/data.go

Lines 286 to 290 in 8e5e54f

	rpm.Value = &RemoteProducerMetadata_TraceData_{
	TraceData: &RemoteProducerMetadata_TraceData{
	CollectedSpans: meta.TraceData,
	},
	}

It hopefully also does propagate errors.

While introducing always-on tracing, we want to spend some time making sure that errors in fact do correctly propagate through traces at all times via the introduction of suitable tests where they do not yet exist.

Jira issue: CRDB-3655