Add fc_trace_tag field to FunctionOutput

[thecodingwizard]

This PR adds a fc_trace_tag field to FunctionGetOutputsItem, needed to provide end-to-end function call traces with Datdog.

We sample a very small number of function calls, and this field is only set for function calls we do sample, so this shouldn't actually impact many outputs.

This tag might also be useful for synmons, etc.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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