[improve][pip] PIP-407 Add a newMessage API to create a message with a schema and transaction

Discussion email: 
Implementation PR: [improve][pip] PIP-406: Introduce pulsar_subscription_dispatch_throttled_msgs and bytes metrics #23945

Author: Ómar Kjartan Yasin

pip/pip-407.md

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+# PIP-407: Add a newMessage API to create a message with a schema and transaction
+
+# Background knowledge
+
+Pulsar allows messages to be produced with a schema and with a transaction. The producer API allows for messages to
+be created with a schema or with a transaction.
+
+# Motivation
+
+The producer API does not expose a method to publish a message with a schema and transaction. This is a gap in the
+public API that can be addressed by adding a new overload that creates a message with a schema and a transaction.
+
+# Goals
+
+Make it possible to produce a message that has a custom schema within a transaction.
+
+## In Scope
+
+Add a new method to the Producer public API, and corresponding implementation, that allows for messages to be created
+with a custom schema and a transaction.
+
+## Public-facing Changes
+
+Add a new method to Producer to create a Message that accepts a schema and transaction
+
+### Public API
+
+#### Producer.java
+
+```java
+    <V> TypedMessageBuilder<V> newMessage(Schema<V> schema,
+                                          Transaction txn);
+```
+
+# Backward & Forward Compatibility
+
+Fully compatible.
+
+# Links
+
+* Mailing List discussion thread: https://lists.apache.org/thread/f8n0fbjj67cpzprxd8qp582ncy7hwtdn
+* Mailing List voting thread: