[processor/metricsgeneration] Add config validation for generated metric names (open-telemetry#37599)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
If a generated metric name matched the metric being scaled an infinite
loop is hit. Since this action is not supported by this processor, and
is supported by the metrics transform processor, the fix here is to add
config validation to enforce metric names are different.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#37474

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added a test to ensure enforcement is done properly.

Author: crobert-1

.chloggen/metricsgeneration_loop.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: metricsgenerationprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Generated metric name may not match metric being scaled
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37474]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/metricsgenerationprocessor/config.go

@@ -161,6 +161,12 @@ func (config *Config) Validate() error {
 		if rule.Operation != "" && !rule.Operation.isValid() {
 			return fmt.Errorf("%q must be in %q", operationFieldName, operationTypeKeys())
 		}
+
+		if rule.Name == rule.Metric1 {
+			return fmt.Errorf("value of field %q may not match value of field %q", nameFieldName, metric1FieldName)
+		} else if rule.Name == rule.Metric2 {
+			return fmt.Errorf("value of field %q may not match value of field %q", nameFieldName, metric2FieldName)
+		}
 	}
 	return nil
 }

processor/metricsgenerationprocessor/config_test.go

@@ -75,6 +75,14 @@ func TestLoadConfig(t *testing.T) {
 			id:           component.NewIDWithName(metadata.Type, "invalid_operation"),
 			errorMessage: fmt.Sprintf("%q must be in %q", operationFieldName, operationTypeKeys()),
 		},
+		{
+			id:           component.NewIDWithName(metadata.Type, "matching_metric1"),
+			errorMessage: fmt.Sprintf("value of field %q may not match value of field %q", nameFieldName, metric1FieldName),
+		},
+		{
+			id:           component.NewIDWithName(metadata.Type, "matching_metric2"),
+			errorMessage: fmt.Sprintf("value of field %q may not match value of field %q", nameFieldName, metric2FieldName),
+		},
 	}
 
 	for _, tt := range tests {

processor/metricsgenerationprocessor/testdata/config.yaml

@@ -68,3 +68,19 @@ metricsgeneration/missing_type:
       metric1: metric1
       metric2: metric2
       operation: percent
+
+metricsgeneration/matching_metric1:
+  rules:
+    - name: new_metric
+      type: scale
+      metric1: new_metric
+      operation: multiply
+      scale_by: 100
+
+metricsgeneration/matching_metric2:
+  rules:
+    - name: new_metric
+      type: calculate
+      metric1: original
+      metric2: new_metric
+      operation: multiply