Add json:"" marshaling annotations to response structs (#21)

## Problem
There's been some UX feedback regarding marshaling of some of our
response structs.

Primarily, we're not using `json:"key_name"` annotation for our struct
fields so they default to capitalized keys in JSON output when
marshaled. IE: `{ "Usage": { "ReadUnits": 5 }}`. We'd like these lower
case and matched with our API spec.

There are also instances where we're returning pointers for certain
`*int32` fields which can be irritating to consumers needing to
dereference and nil-check. In certain cases we can default any nil
pointer values to 0 before returning things.

There's no testing for the marshaling on any of our custom structs.

## Solution
I may have gone a bit overboard with the testing here. The bulk of this
is tests and the tests are all very similar, definitely open to opinions
on removing if people find them unnecessary, but changing up how the
JSON is produced is pretty easy and it's an important point of
integration in my mind, so maybe worth testing for.

I also wasn't clear if adding these annotations to our request structs
made sense either, so I left those out for now. Overall I'm still
working on getting a better understanding of Go syntax and expectations.

- Add JSON marshaling annotations to the response structs in `models.go`
and `index_connection.go`.
- Update `Collection` and `Usage` structs to default integer values
rather than using pointers.
- Add new `models_test.go` file to run through some validation on struct
marshaling, also add unit tests to `index_connection_test.go`.

Originally I thought `omitempty` for some of these pointers may be
helpful from a consumer standpoint, but I'm actually not sure the more
I've thought about it. Does it make sense to omit more or less things?
Other feedback here would be much appreciated.

## Type of Change
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [X] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan
Unit tests to explicitly stress marshaling the structs via
`json.Marshal()`.

pinecone/client.go

@@ -386,12 +386,13 @@ func toCollection(cm *control.CollectionModel) *Collection {
 	if cm == nil {
 		return nil
 	}
+
 	return &Collection{
 		Name:        cm.Name,
-		Size:        cm.Size,
+		Size:        derefOrDefault(cm.Size, 0),
 		Status:      CollectionStatus(cm.Status),
-		Dimension:   cm.Dimension,
-		VectorCount: cm.VectorCount,
+		Dimension:   derefOrDefault(cm.Dimension, 0),
+		VectorCount: derefOrDefault(cm.VectorCount, 0),
 		Environment: cm.Environment,
 	}
 }
@@ -413,6 +414,13 @@ func minOne(x int32) int32 {
 	return x
 }
 
+func derefOrDefault[T any](ptr *T, defaultValue T) T {
+	if ptr == nil {
+		return defaultValue
+	}
+	return *ptr
+}
+
 func buildClientOptions(in NewClientParams) ([]control.ClientOption, error) {
 	clientOptions := []control.ClientOption{}
 	hasAuthorizationHeader := false

pinecone/index_connection.go

@@ -14,18 +14,18 @@ import (
 )
 
 type IndexConnection struct {
-	Namespace  string
-	apiKey     string
+	Namespace          string
+	apiKey             string
 	additionalMetadata map[string]string
-	dataClient *data.VectorServiceClient
-	grpcConn   *grpc.ClientConn
+	dataClient         *data.VectorServiceClient
+	grpcConn           *grpc.ClientConn
 }
 
 type newIndexParameters struct {
-	apiKey string
-	host string
-	namespace string
-	sourceTag string
+	apiKey             string
+	host               string
+	namespace          string
+	sourceTag          string
 	additionalMetadata map[string]string
 }
 
@@ -75,8 +75,8 @@ func (idx *IndexConnection) UpsertVectors(ctx context.Context, in []*Vector) (ui
 }
 
 type FetchVectorsResponse struct {
-	Vectors map[string]*Vector
-	Usage   *Usage
+	Vectors map[string]*Vector `json:"vectors,omitempty"`
+	Usage   *Usage             `json:"usage,omitempty"`
 }
 
 func (idx *IndexConnection) FetchVectors(ctx context.Context, ids []string) (*FetchVectorsResponse, error) {
@@ -94,6 +94,7 @@ func (idx *IndexConnection) FetchVectors(ctx context.Context, ids []string) (*Fe
 	for id, vector := range res.Vectors {
 		vectors[id] = toVector(vector)
 	}
+	fmt.Printf("VECTORS: %+v\n", vectors)
 
 	return &FetchVectorsResponse{
 		Vectors: vectors,
@@ -108,9 +109,9 @@ type ListVectorsRequest struct {
 }
 
 type ListVectorsResponse struct {
-	VectorIds           []*string
-	Usage               *Usage
-	NextPaginationToken *string
+	VectorIds           []*string `json:"vector_ids,omitempty"`
+	Usage               *Usage    `json:"usage,omitempty"`
+	NextPaginationToken *string   `json:"next_pagination_token,omitempty"`
 }
 
 func (idx *IndexConnection) ListVectors(ctx context.Context, in *ListVectorsRequest) (*ListVectorsResponse, error) {
@@ -132,7 +133,7 @@ func (idx *IndexConnection) ListVectors(ctx context.Context, in *ListVectorsRequ
 
 	return &ListVectorsResponse{
 		VectorIds:           vectorIds,
-		Usage:               &Usage{ReadUnits: res.Usage.ReadUnits},
+		Usage:               &Usage{ReadUnits: derefOrDefault(res.Usage.ReadUnits, 0)},
 		NextPaginationToken: toPaginationToken(res.Pagination),
 	}, nil
 }
@@ -147,8 +148,8 @@ type QueryByVectorValuesRequest struct {
 }
 
 type QueryVectorsResponse struct {
-	Matches []*ScoredVector
-	Usage   *Usage
+	Matches []*ScoredVector `json:"matches,omitempty"`
+	Usage   *Usage          `json:"usage,omitempty"`
 }
 
 func (idx *IndexConnection) QueryByVectorValues(ctx context.Context, in *QueryByVectorValuesRequest) (*QueryVectorsResponse, error) {
@@ -236,10 +237,10 @@ func (idx *IndexConnection) UpdateVector(ctx context.Context, in *UpdateVectorRe
 }
 
 type DescribeIndexStatsResponse struct {
-	Dimension        uint32
-	IndexFullness    float32
-	TotalVectorCount uint32
-	Namespaces       map[string]*NamespaceSummary
+	Dimension        uint32                       `json:"dimension"`
+	IndexFullness    float32                      `json:"index_fullness"`
+	TotalVectorCount uint32                       `json:"total_vector_count"`
+	Namespaces       map[string]*NamespaceSummary `json:"namespaces,omitempty"`
 }
 
 func (idx *IndexConnection) DescribeIndexStats(ctx context.Context) (*DescribeIndexStatsResponse, error) {
@@ -335,7 +336,7 @@ func toUsage(u *data.Usage) *Usage {
 		return nil
 	}
 	return &Usage{
-		ReadUnits: u.ReadUnits,
+		ReadUnits: derefOrDefault(u.ReadUnits, 0),
 	}
 }
 
@@ -372,7 +373,7 @@ func (idx *IndexConnection) akCtx(ctx context.Context) context.Context {
 	newMetadata := []string{}
 	newMetadata = append(newMetadata, "api-key", idx.apiKey)
 
-	for key, value := range idx.additionalMetadata{
+	for key, value := range idx.additionalMetadata {
 		newMetadata = append(newMetadata, key, value)
 	}