[profiles] Clarify the original payload field comments. (#722)

* Clarify the original payload field comments.

Author: aalexand

CHANGELOG.md

@@ -6,6 +6,7 @@ The full list of changes can be found in the compare view for the respective rel
 
 ### Changed
 
+- profiles: clarify the original payload field comments. [#722](https://github.com/open-telemetry/opentelemetry-proto/pull/722)
 - profiles: add a note about cardinality implications for attribute values. [#713](https://github.com/open-telemetry/opentelemetry-proto/pull/713)
 - profiles: rename line -> lines and sample -> samples since they are repeated fields. [#712](https://github.com/open-telemetry/opentelemetry-proto/pull/712)
 

opentelemetry/proto/profiles/v1development/profiles.proto

@@ -305,15 +305,28 @@ message Profile {
   // attributes. If this value is 0, then no attributes were dropped.
   uint32 dropped_attributes_count = 9;
 
-  // Specifies format of the original payload. Common values are defined in semantic conventions. [required if original_payload is present]
+  // The original payload format. See also original_payload. Optional, but the
+  // format and the bytes must be set or unset together.
+  //
+  // The allowed values for the format string are defined by the OpenTelemetry
+  // specification. Some examples are "jfr", "pprof", "linux_perf".
+  //
+  // The original payload may be optionally provided when the conversion to the
+  // OLTP format was done from a different format with some loss of the fidelity
+  // and the receiver may want to store the original payload to allow future
+  // lossless export or reinterpretation. Some examples of the original format
+  // are JFR (Java Flight Recorder), pprof, Linux perf.
+  //
+  // Even when the original payload is in a format that is semantically close to
+  // OTLP, such as pprof, a conversion may still be lossy in some cases (e.g. if
+  // the pprof file contains custom extensions or conventions).
+  //
+  // The original payload can be large in size, so including the original
+  // payload should be configurable by the profiler or collector options. The
+  // default behavior should be to not include the original payload.
   string original_payload_format = 10;
-
-  // Original payload can be stored in this field. This can be useful for users who want to get the original payload.
-  // Formats such as JFR are highly extensible and can contain more information than what is defined in this spec.
-  // Inclusion of original payload should be configurable by the user. Default behavior should be to not include the original payload.
-  // If the original payload is in pprof format, it SHOULD not be included in this field.
-  // The field is optional, however if it is present then equivalent converted data should be populated in other fields
-  // of this message as far as is practicable.
+  // The original payload bytes. See also original_payload_format. Optional, but
+  // format and the bytes must be set or unset together.
   bytes original_payload = 11;
 
   // References to attributes in attribute_table. [optional]