[ServiceBus] Track2 - Re-expose AMQP details on message

[KieranBrantnerMagee]

Given the ask to allow access to AMQP properties such as annotations and headers, there are two proposals at hand.

Proposal 1
Flat object.

Pros:

• fewer new object concepts
• less depth
• all props immediately visible.

Cons:

• all props immediately visible.
• amqp/non-amqp confusion at top level

class PeekedMessage(Message):
    def __init__(self, message : AMQPMessage, ...):
        ...
        self.annotations = message.annotations # type: dict[str,str]
        self.headers = message.annotations # type: dict[str,str]
        ...

# accessed as:
message.annotations["foo"]="bar"

Sub-Proposal 1-B

As above, but amqp_[headers, annotations, etc] naming.

Proposal 2
AMQP sub-object. echoes the design of "ServiceBusReceiver.Session" for function-specific grouping if niche functionality.

Pros:

• logical encapsulation, more communicative.
• keeps core message surface area cleaner.

Cons:

• additional layer of depth/additional concept.
• if we ever introduce circular dependencies between these props, (e.g. A sets B, B sets A) it'll be problematic.

class AMQPMessageProperties(Object):
    def __init__(self, annotations, headers, ...)
        ...
        self.annotations = annotations # type: dict[str,str]
        self.headers = annotations  # type: dict[str,str]
        ...

class PeekedMessage(Message):
    def __init__(self, message : AMQPMessage, ...):
        ...
        self.amqp = AMQPMessageProperties(message.annotations, message.headers, ...) # type: AMQPMessageProperties
        ...

# accessed as:
message.amqp.annotations["foo"] = "bar"