[mdatagen] bug fix - reaggreagate_set test generation (#14441)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Fixes reaggregation test generation when metrics have attributes with
`requirement_level: required`.

1. **Config validation failure**: The generated `testdata/config.yaml`
included `attributes: []` in the `reaggregate_set` section, but required
attributes must always be present. This caused config validation to fail
with an error like:
```
error decoding 'metrics': attributes [required_string_attr] are required for metric reaggregate.metric.with_required
```
**Fix**: Updated `config.yaml.tmpl` to use `requiredAttributes` helper
to include required attributes in the `reaggregate_set` config instead
of an empty array.

2. **Test assertion failure**: The generated `metrics_test.go` used
different values for required attributes across data points, preventing
them from merging during reaggregation. The test expected 1 data point
but received 2.

**Fix** - Updated `metrics_test.go.tmpl` to use the same value for
required attributes across both data points in reaggregation tests,
allowing them to merge correctly while still testing that non-required
attributes are dropped.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes #

<!--Describe what testing was performed and which tests were added.-->
#### Testing
- Added `required_string_attr` attribute and
`reaggregate.metric.with_required` metric to
`samplereceiver/metadata.yaml` to cover this scenario

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Author: jinja2

cmd/mdatagen/internal/loader_test.go

@@ -244,6 +244,14 @@ func TestLoadMetadata(t *testing.T) {
 						FullName:         "opt_in_bool_attr",
 						RequirementLevel: AttributeRequirementLevelOptIn,
 					},
+					"required_string_attr": {
+						Description: "A required attribute with a string value",
+						Type: ValueType{
+							ValueType: pcommon.ValueTypeStr,
+						},
+						FullName:         "required_string_attr",
+						RequirementLevel: AttributeRequirementLevelRequired,
+					},
 				},
 				Metrics: map[MetricName]Metric{
 					"default.metric": {
@@ -267,7 +275,7 @@ func TestLoadMetadata(t *testing.T) {
 					"reaggregate.metric": {
 						Signal: Signal{
 							Enabled:     true,
-							Description: "Metric for testing spacial reaggregation",
+							Description: "Metric for testing spatial reaggregation",
 							Stability:   Stability{Level: component.StabilityLevelBeta},
 							Attributes:  []AttributeName{"string_attr", "boolean_attr"},
 						},
@@ -276,6 +284,18 @@ func TestLoadMetadata(t *testing.T) {
 							MetricValueType: MetricValueType{pmetric.NumberDataPointValueTypeDouble},
 						},
 					},
+					"reaggregate.metric.with_required": {
+						Signal: Signal{
+							Enabled:     true,
+							Description: "Metric for testing spatial reaggregation with required attributes",
+							Stability:   Stability{Level: component.StabilityLevelBeta},
+							Attributes:  []AttributeName{"required_string_attr", "string_attr", "boolean_attr"},
+						},
+						Unit: strPtr("1"),
+						Gauge: &Gauge{
+							MetricValueType: MetricValueType{pmetric.NumberDataPointValueTypeDouble},
+						},
+					},
 					"system.cpu.time": {
 						Signal: Signal{
 							Enabled:               true,

cmd/mdatagen/internal/samplereceiver/documentation.md

@@ -65,7 +65,7 @@ Monotonic cumulative sum int metric with string input_type enabled by default.
 
 ### reaggregate.metric
 
-Metric for testing spacial reaggregation
+Metric for testing spatial reaggregation
 
 | Unit | Metric Type | Value Type | Stability |
 | ---- | ----------- | ---------- | --------- |
@@ -78,6 +78,22 @@ Metric for testing spacial reaggregation
 | string_attr | Attribute with any string value. | Any Str | Recommended |
 | boolean_attr | Attribute with a boolean value. | Any Bool | Recommended |
 
+### reaggregate.metric.with_required
+
+Metric for testing spatial reaggregation with required attributes
+
+| Unit | Metric Type | Value Type | Stability |
+| ---- | ----------- | ---------- | --------- |
+| 1 | Gauge | Double | Beta |
+
+#### Attributes
+
+| Name | Description | Values | Requirement Level |
+| ---- | ----------- | ------ | -------- |
+| required_string_attr | A required attribute with a string value | Any Str | Required |
+| string_attr | Attribute with any string value. | Any Str | Recommended |
+| boolean_attr | Attribute with a boolean value. | Any Bool | Recommended |
+
 ### system.cpu.time
 
 Monotonic cumulative sum int metric enabled by default.