fix: Go SDK metrics unmarshal error with OptionalNullable[time.Time]

[alexluong]

Summary

Fixes Go SDK crash when calling GetEventMetrics or GetAttemptMetrics with granularity:

json: cannot unmarshal string into Go value of type map[string]json.RawMessage

Root cause

Speakeasy's generated unmarshalValue in json.go has a bug with OptionalNullable[time.Time]. The type is map[bool]*time.Time under the hood. When unmarshalling a map field, the code checks if the map's value type is a "complex value type" (time.Time, big.Int, types.Date). If it is, it skips the standard json.Unmarshal path (which would correctly call OptionalNullable.UnmarshalJSON()) and falls through to custom map handling that tries to unmarshal a datetime string into map[string]json.RawMessage — which crashes.

This only affects OptionalNullable[time.Time]. Other OptionalNullable types like OptionalNullable[string] work fine because string is not a "complex value type". The marshal side already handles this correctly with a json.Marshaler check, but the unmarshal side is missing the equivalent json.Unmarshaler check. This is a Speakeasy codegen bug — we'll report it upstream.

Fix

• OpenAPI spec: Changed time_bucket and granularity from nullable (type: [string, "null"]) to non-nullable (type: string). This makes Speakeasy generate *time.Time / *string instead of OptionalNullable[T], avoiding the buggy code path.
• Server: Added omitempty to APIMetricsDataPoint.TimeBucket and APIMetricsMetadata.Granularity JSON tags so the server omits these fields when nil instead of returning null. This matches the updated spec.

API behavior change

When no granularity is specified, the response changes from:

{
  "data": [{"time_bucket": null, "dimensions": {}, "metrics": {"count": 100}}],
  "metadata": {"granularity": null, "query_time_ms": 5, ...}
}

to:

{
  "data": [{"dimensions": {}, "metrics": {"count": 100}}],
  "metadata": {"query_time_ms": 5, ...}
}

The fields are omitted instead of sent as null. Functionally equivalent for consumers — both result in nil/undefined when accessed.

Test plan

• Regression test reproduces the original error before fix
• SDK test passes after spec change (with value, absent, and null cases)
• Server-side metrics tests pass with omitempty change
• Speakeasy codegen will regenerate the SDK on merge

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