feat: source-transformer-map-sink spans

[adarsh0728]

What this PR does / why we need it

• add a per-message numaflow.platform.process root span
• add numaflow.monovertex.source.dispatch to measure source-stage work
• add numaflow.monovertex.source.transform for source-transformer UDF time
• keep numaflow.monovertex.map and numaflow.monovertex.sink.write under the same root span
• handle source-transformer fan-out correctly so one input that emits N outputs is traced

Fan-out Handling

This PR explicitly handles the source-transformer 1->N case:

• platform.process is still created once per source input.
• source.transform is still created once per source input.
• source.dispatch stays open until the final emitted child message for that source input is dispatched/bypassed downstream.
• The zero-output/filter case is handled as well by closing the dispatch span immediately after transform.

Note

• numaflow.platform.process created in source once a message exists, i.e. after Self::read() returns and before tracker insertion.
• PR does not create a per-message source.read span.
• Per-message spans can only be created after the batch read returns
• source.dispatch operation name can be changed if required.

Related issues

Fixes #3379

Testing

In Progress

Special notes for reviewers

Will change the base to main once tracing-base branch PR is merged

[vigith]

you do not require this because i have added a drop guard on the other one.

[adarsh0728]

Will create a new PR.