[mdatagen] Add ability to specify additional warnings in metadata.yaml (

open-telemetry#17180)

- `if_enabled_not_set`: to require `enabled` field to be explicitly set by users in case the metric is going to be turned from optional to default and vice versa.

- `if_configured`: to ensure that users do not configure the metric. It should be used for deprecated optional metrics that will be removed soon.

This change also adds more test coverage for the metrics config unmarshaling.

.chloggen/mdatagen-more-warnings.yaml

@@ -0,0 +1,11 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: mdatagen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add ability to specify additional warnings in metadata.yaml
+
+# One or more tracking issues related to the change
+issues: [17180]

cmd/mdatagen/documentation.md

@@ -16,7 +16,7 @@ metrics:
 
 Monotonic cumulative sum int metric enabled by default.
 
-Additional information.
+The metric will be become optional soon.
 
 | Unit | Metric Type | Value Type | Aggregation Temporality | Monotonic |
 | ---- | ----------- | ---------- | ----------------------- | --------- |
@@ -30,6 +30,16 @@ Additional information.
 | state | Integer attribute with overridden name. | Any Int |
 | enum_attr | Attribute with a known set of string values. | Str: ``red``, ``green``, ``blue`` |
 
+### default.metric.to_be_removed
+
+[DEPRECATED] Non-monotonic delta sum double metric enabled by default.
+
+The metric will be will be removed soon.
+
+| Unit | Metric Type | Value Type | Aggregation Temporality | Monotonic |
+| ---- | ----------- | ---------- | ----------------------- | --------- |
+| s | Sum | Double | Delta | false |
+
 ## Optional Metrics
 
 The following metrics are not emitted by default. Each of them can be enabled by applying the following configuration:

cmd/mdatagen/internal/metadata/testdata/config.yaml

@@ -0,0 +1,15 @@
+default:
+all_metrics:
+  default.metric:
+    enabled: true
+  default.metric.to_be_removed:
+    enabled: true
+  optional.metric:
+    enabled: true
+no_metrics:
+  default.metric:
+    enabled: false
+  default.metric.to_be_removed:
+    enabled: false
+  optional.metric:
+    enabled: false

cmd/mdatagen/loader.go

@@ -175,6 +175,10 @@ func (m metric) Data() MetricData {
 type warnings struct {
 	// A warning that will be displayed if the metric is enabled in user config.
 	IfEnabled string `mapstructure:"if_enabled"`
+	// A warning that will be displayed if `enabled` field is not set explicitly in user config.
+	IfEnabledNotSet string `mapstructure:"if_enabled_not_set"`
+	// A warning that will be displayed if the metrics is configured by user in any way.
+	IfConfigured string `mapstructure:"if_configured"`
 }
 
 type attribute struct {