[1/n] add application level autoscaling policy in schema

[abrarsheikh]

part 1 of #56149

1. move _serialized_policy_def into AutoscalingPolicy from AutoscalingConfig. We need this in order to reuse AutoscalingPolicy for application-level autoscaling.
2. Make autoscaling_policy a top-level config in ServeApplicationSchema.

[gemini-code-assist]

Code Review

This pull request refactors the autoscaling policy configuration by moving the serialization logic from AutoscalingConfig into AutoscalingPolicy. It also introduces an application-level autoscaling_policy in ServeApplicationSchema. The changes are logical and improve encapsulation.

I've identified a critical issue in the protobuf schema update that breaks wire compatibility, and a medium-severity issue regarding schema definition in ServeApplicationSchema. Please see the detailed comments.

[gemini-code-assist]

Renumbering protobuf fields is a wire-incompatible breaking change. An older component serializing an AutoscalingConfig message will use field numbers 12, 13, etc., for policy, target_ongoing_requests, etc. A newer component will interpret these as target_ongoing_requests, upscaling_factor, etc., leading to silent data corruption or deserialization errors.

To maintain compatibility, you should not renumber the existing fields. Please revert the field numbers to their original values. It's also a good practice to mark the removed field number as reserved to prevent accidental reuse, like this:

...
optional double downscale_smoothing_factor = 10;

reserved 11;

// The autoscaling policy definition.
AutoscalingPolicy policy = 12;
...

Suggested change

	// The autoscaling policy definition.
	AutoscalingPolicy policy = 12;
	AutoscalingPolicy policy = 11;
	// Target number of in flight requests per replica. This is the primary configuration
	// knob for replica autoscaler. Lower the number, the more rapidly the replicas
	// scales up. Must be a non-negative integer.
	double target_ongoing_requests = 13;
	double target_ongoing_requests = 12;
	// The multiplicative "gain" factor to limit upscale.
	optional double upscaling_factor = 14;
	optional double upscaling_factor = 13;
	// The multiplicative "gain" factor to limit downscale.
	optional double downscaling_factor = 15;
	optional double downscaling_factor = 14;
	// How long to wait before scaling down replicas from 1 to 0
	optional double downscale_to_zero_delay_s = 16;
	optional double downscale_to_zero_delay_s = 15;
	// The autoscaling policy definition.
	AutoscalingPolicy policy = 12;
	// Target number of in flight requests per replica. This is the primary configuration
	// knob for replica autoscaler. Lower the number, the more rapidly the replicas
	// scales up. Must be a non-negative integer.
	double target_ongoing_requests = 13;
	// The multiplicative "gain" factor to limit upscale.
	optional double upscaling_factor = 14;
	// The multiplicative "gain" factor to limit downscale.
	optional double downscaling_factor = 15;
	// How long to wait before scaling down replicas from 1 to 0
	optional double downscale_to_zero_delay_s = 16;

[harshit-anyscale]

+1, we shouldn't numbers assigned to existing fields, this can make the code backward incompatible

[abrarsheikh]

this is not a problem because we dont store these proto records in DB and use it in different session

[gemini-code-assist]

The type hint and default value for autoscaling_policy are inconsistent with its description. The description mentions "If null...", which implies the field should be optional and can be None.

1. The type hint should be Optional[dict] to allow None as a value.
2. The default value should be None to match the "if null" condition described.
3. Using default={} creates a mutable default value, which can lead to unexpected behavior. It's better to use default=None or default_factory=dict. In this case, default=None is the most clear and correct choice based on the description.

Suggested change

	autoscaling_policy: dict = Field(
	default={},
	description=(
	"Autoscaling policy for the application. "
	"If null, serve fallbacks to autoscaling policy in each deployment. "
	"This option is under development and not yet supported."
	),
	)
	autoscaling_policy: Optional[dict] = Field(
	default=None,
	description=(
	"Autoscaling policy for the application. "
	"If null, serve fallbacks to autoscaling policy in each deployment. "
	"This option is under development and not yet supported."
	),
	)

[zcin]

@abrarsheikh merge conflicts

[Kishanthan]

LGTM