Document more explicitly the dictionary zero element requirement. (#711)

Right now we have a comment to the dictionary proto and a comment to the
string_table field. This requirement is important enough I think that
it's worth documenting it more explicitly, especially the part that even
if there are no references to the particular dictionary, the zero index
element must still be present.

Author: aalexand

opentelemetry/proto/profiles/v1development/profiles.proto

@@ -106,19 +106,28 @@ option go_package = "go.opentelemetry.io/proto/otlp/profiles/v1development";
 message ProfilesDictionary {
   // Mappings from address ranges to the image/binary/library mapped
   // into that address range referenced by locations via Location.mapping_index.
+  //
+  // mapping_table[0] must always be zero value (Mapping{}) and present.
   repeated Mapping mapping_table = 1;
 
   // Locations referenced by samples via Stack.location_indices.
+  //
+  // location_table[0] must always be zero value (Location{}) and present.
   repeated Location location_table = 2;
 
   // Functions referenced by locations via Line.function_index.
+  //
+  // function_table[0] must always be zero value (Function{}) and present.
   repeated Function function_table = 3;
 
   // Links referenced by samples via Sample.link_index.
+  //
+  // link_table[0] must always be zero value (Link{}) and present.
   repeated Link link_table = 4;
 
   // A common table for strings referenced by various messages.
-  // string_table[0] must always be "".
+  //
+  // string_table[0] must always be "" and present.
   repeated string string_table = 5;
 
   // A common table for attributes referenced by various messages.
@@ -139,9 +148,13 @@ message ProfilesDictionary {
   // These restrictions can change in a minor release.
   // The restrictions take origin from the OpenTelemetry specification:
   // https://github.com/open-telemetry/opentelemetry-specification/blob/v1.47.0/specification/common/README.md#attribute.
+  //
+  // attribute_table[0] must always be zero value (KeyValueAndUnit{}) and present.
   repeated KeyValueAndUnit attribute_table = 6;
 
   // Stacks referenced by samples via Sample.stack_index.
+  //
+  // stack_table[0] must always be zero value (Stack{}) and present.
   repeated Stack stack_table = 7;
 }