Introduce specification for OTel Profiles (#4197)

## Changes

Proposed change along with open-telemetry/semantic-conventions#1329 to clarify the use of `build_id` in the context of OTel Profiles.

FYI: @open-telemetry/profiling-maintainers 

* [x] [`CHANGELOG.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CHANGELOG.md) file updated for non-trivial changes

Author: florianl

CHANGELOG.md

@@ -24,6 +24,11 @@ release.
 
 ### Resource
 
+### Profiles
+
+- Define required attributes for Mappings.
+  ([#4197](https://github.com/open-telemetry/opentelemetry-specification/pull/4197))
+
 ### OpenTelemetry Protocol
 
 ### Compatibility

specification/README.md

@@ -42,6 +42,7 @@ path_base_for_github_subdir:
   - [Protocol](protocol/README.md)
     - [Metrics](metrics/data-model.md)
     - [Logs](logs/data-model.md)
+    - [Profiles](profiles/mappings.md)
   - Compatibility
     - [OpenCensus](compatibility/opencensus.md)
     - [OpenTracing](compatibility/opentracing.md)

specification/profiles/README.md

@@ -0,0 +1,7 @@
+<!--- Hugo front matter used to generate the website version of this page:
+path_base_for_github_subdir:
+  from: tmp/otel/specification/profiles/_index.md
+  to: profiles/README.md
+--->
+
+# Profiles

specification/profiles/mappings.md

@@ -0,0 +1,36 @@
+# Mappings
+
+**Status**: [Development](../document-status.md)
+
+This document defines the required attributes of [`Mapping`](https://github.com/open-telemetry/oteps/blob/main/text/profiles/0239-profiles-data-model.md#message-mapping) messages.
+
+<!-- toc -->
+
+- [Attributes](#attributes)
+  * [Algorithm for `process.executable.build_id.htlhash`](#algorithm-for-processexecutablebuild_idhtlhash)
+
+<!-- tocstop -->
+
+## Attributes
+
+A message representing a `Mapping` MUST have at least one of the following
+[process attributes](https://opentelemetry.io/docs/specs/semconv/attributes-registry/process/#process-attributes):
+
+- `process.executable.build_id.gnu`
+- `process.executable.build_id.go`
+- `process.executable.build_id.htlhash`
+
+If possible all the above listed attributes SHOULD be present in a `Mapping`. To promote interoperability, it is RECOMMENDED for `process.executable.build_id.htlhash` to be present in every `Mapping`. For the use and purpose of `process.executable.build_id.go` see [golang/go#68652](https://github.com/golang/go/issues/68652#issuecomment-2274452424).
+
+### Algorithm for `process.executable.build_id.htlhash`
+
+In some environments GNU and/or Go build_id values are stripped or not usable - for example Alpine
+Linux which is often used as a base for Docker environments. For that reason and to promote interoperability, a deterministic build_id generation algorithm that hashes the first and last page of a file together with its length is defined as:
+
+```
+Input   ← Concat(File[:4096], File[-4096:], BigEndianUInt64(Len(File)))
+Digest  ← SHA256(Input)
+BuildID ← Digest[:16]
+```
+
+where `Input` is the concatenation of the first and last 4096 bytes of the file (may overlap, not padded) and the 8 byte big-endian serialization of the file length. The resulting `BuildID` is the truncation of the hash digest to 16 bytes (128 bits), in hex string form.