feat(firestore): [PQ] Add all Aggregation and Timestamp functions

[bhshkh]

b/364927702

1. add all the remaining private preview aggregate functions. Merging this PR completes the implementation of all the type: "Function" subType : "Accumulators (Aggregation)" private preview features.
   See "Firestore Features (Pipeline)" sheet in go/firestore-query-tracker for the list of features.

   Java reference:

   • https://github.com/googleapis/java-firestore/blob/ccaf9d4fac5bd87a4da3d37493ca66fdc7681bc3/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/AggregateFunction.java#L43-L71
   • https://github.com/googleapis/java-firestore/blob/ccaf9d4fac5bd87a4da3d37493ca66fdc7681bc3/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/AggregateFunction.java#L93-L111

2. add all the remaining private preview timestamp functions. Merging this PR completes the implementation of all the type: "Function" subType: "Date / Timestamp" private preview features. (except timestamp_trunc function which is not yet inmplemented in any of the SDKs. Requires additonal approvals from Firestore team and will be added to separate PR).
   See "Firestore Features (Pipeline)" sheet in go/firestore-query-tracker for the list of functions.

   Java reference:

   • https://github.com/googleapis/java-firestore/blob/ccaf9d4fac5bd87a4da3d37493ca66fdc7681bc3/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/Expression.java#L2262-L2517

3. Add integration tests for all functions.

4. Remove Rand function since it is not targeted for private preview.

5. Renamed numericExprOrField to numericExprOrFieldPath since field is a separate type/expression.
   

   google-cloud-go/firestore/pipeline_field.go

   Lines 21 to 41 in a3ee1f1

   	// field represents a reference to a field in a Firestore document, or outputs of a [Pipeline] stage.
   	// It implements the [Expr] and [Selectable] interfaces.
   	//
   	// Field references are used to access document field values in expressions and to specify fields
   	// for sorting, filtering, and projecting data in Firestore pipelines.
   	type field struct {
   	*baseExpr
   	fieldPath FieldPath
   	}
   	// FieldOf creates a new field [Expr] from a field path string.
   	func FieldOf(path string) Expr {
   	fieldPath, err := parseDotSeparatedString(path)
   	if err != nil {
   	return &field{baseExpr: &baseExpr{err: err}}
   	}
   	return FieldOfPath(fieldPath)
   	}
   	// FieldOfPath creates a new field [Expr] for the given [FieldPath].
   	func FieldOfPath(fieldPath FieldPath) Expr {

Previous pull requests

• feat(firestore): [PQ] add pipeline queries #12217
• feat(firestore): [PQ] Add rest of the input stages #12425
• feat(firestore): [PQ] Add pipeline expressions basic structure #12538
• feat(firestore): [PQ] add all the remaining arithmetic and comparison functions #13147
• test(firestore): [PQ] Modify integration test to run on enterprise db #13199
• feat(firestore): [PQ] add all the remaining private preview stages #13218

[bhshkh]

/gemini review

[gemini-code-assist]

Code Review

This pull request completes the implementation of private preview aggregation and timestamp functions for Firestore pipelines. It adds several new functions like Sum, Average, Count, Maximum, Minimum, various timestamp manipulation functions, and their corresponding integration tests. The changes are well-structured, and the addition of tests is great. I've found a few minor issues, mostly related to documentation copy-paste errors in the new aggregate functions and some leftover code in tests. There is also one change that looks like a temporary debug modification that should probably be reverted before merging.

[daniel-sanche]

LGTM, but left a few comments

[hongalex]

LGTM for Go, want approval for firestore logic