Add comment to explain schema_url fields

Resolves #502

opentelemetry/proto/logs/v1/logs.proto

@@ -55,6 +55,9 @@ message ResourceLogs {
   // A list of ScopeLogs that originate from a resource.
   repeated ScopeLogs scope_logs = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the resource data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to the data in the "resource" field. It does not apply
   // to the data in the "scope_logs" field which have their own schema_url field.
   string schema_url = 3;
@@ -70,6 +73,9 @@ message ScopeLogs {
   // A list of log records.
   repeated LogRecord log_records = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the log data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to all logs in the "logs" field.
   string schema_url = 3;
 }

opentelemetry/proto/metrics/v1/metrics.proto

@@ -55,6 +55,9 @@ message ResourceMetrics {
   // A list of metrics that originate from a resource.
   repeated ScopeMetrics scope_metrics = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the resource data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to the data in the "resource" field. It does not apply
   // to the data in the "scope_metrics" field which have their own schema_url field.
   string schema_url = 3;
@@ -70,6 +73,9 @@ message ScopeMetrics {
   // A list of metrics that originate from an instrumentation library.
   repeated Metric metrics = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the metric data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to all metrics in the "metrics" field.
   string schema_url = 3;
 }

opentelemetry/proto/trace/v1/trace.proto

@@ -55,6 +55,9 @@ message ResourceSpans {
   // A list of ScopeSpans that originate from a resource.
   repeated ScopeSpans scope_spans = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the resource data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to the data in the "resource" field. It does not apply
   // to the data in the "scope_spans" field which have their own schema_url field.
   string schema_url = 3;
@@ -70,6 +73,9 @@ message ScopeSpans {
   // A list of Spans that originate from an instrumentation scope.
   repeated Span spans = 2;
 
+  // The Schema URL, if known. This is the identifier of the Schema that the span data
+  // is recorded in. To learn more about Schema URL see
+  // https://opentelemetry.io/docs/specs/otel/schemas/#schema-url
   // This schema_url applies to all spans and span events in the "spans" field.
   string schema_url = 3;
 }