Update user_events metrics exporter to reflect new schema (#1224)

* cli

* update

* Update mod.rs

* lint

opentelemetry-user-events-metrics/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+### Changed
+
+- Add version, protocol to schema
+    [#1224](https://github.com/open-telemetry/opentelemetry-rust/pull/1224).
+
 ## v0.1.0
 
 ### Added

opentelemetry-user-events-metrics/src/tracepoint/mod.rs

@@ -2,6 +2,11 @@ use core::ffi;
 use eventheader::_internal as ehi;
 use std::pin::Pin;
 
+/// Protocol constant
+const PROTOCOL_FIELD_VALUE: u32 = 0;
+/// Protobuf definition version
+const PROTOBUF_VERSION: &[u8; 8] = b"v0.19.00";
+
 /// This is the command string for the event. It needs to follow the
 /// [Command Format](https://docs.kernel.org/trace/user_events.html#command-format)
 /// syntax, it needs to end with a "\0", and it needs to stay in sync with the
@@ -12,18 +17,21 @@ use std::pin::Pin;
 /// For this event:
 ///
 /// - Event is named "otlp_metrics".
-/// - Field 1 is named "buffer" and has type "variable-length array of u8".
+/// - Field 1 is named "protocol". Value 0 corresponds to protobuf.
+/// - Field 2 is named "version". Corresponds to protocol version (protobuf version).
+/// - Field 3 is named "buffer" and has type "variable-length array of u8".
 ///
 /// "__rel_loc" is a special type for variable-length fields. It requires
 /// special handling in the write() method.
-const METRICS_EVENT_DEF: &[u8] = b"otlp_metrics __rel_loc u8[] buffer\0";
+const METRICS_EVENT_DEF: &[u8] =
+    b"otlp_metrics u32 protocol;char[8] version;__rel_loc u8[] buffer;\0";
 
 /// If the tracepoint is registered and enabled, writes an event. If the tracepoint
 /// is unregistered or disabled, this does nothing and returns 0. You should usually
 /// check [`enabled()`] and only build the buffer and call `write()` if `enabled()`
 /// returns true.
 ///
-/// Requires: buffer.len() < 65536.
+/// Requires: PROTOBUF_VERSION.len() == 8, buffer.len() < 65536.
 ///
 /// Return value is 0 for success or an errno code for error. The return value is
 /// provided to help with debugging and should usually be ignored in release builds.
@@ -35,16 +43,23 @@ pub fn write(trace_point: &ehi::TracepointState, buffer: &[u8]) -> i32 {
         return -1;
     }
 
+    if PROTOBUF_VERSION.len() != 8 {
+        eprintln!("Version must be char[8].");
+        return -1;
+    }
+
     // The rel_loc for the buffer field stores the size and offset of the buffer.
     // - High 16 bits store the size = buffer.len()
     // - Low 16 bits store the offset of the buffer from the end of the rel_loc field = 0.
     let buffer_rel_loc: u32 = (buffer.len() as u32) << 16;
 
     trace_point.write(&mut [
         // mut because the write method does some fix-ups.
-        ehi::EventDataDescriptor::zero(), // First item in array MUST be zero().
+        ehi::EventDataDescriptor::zero(), // First item before buffer MUST be zero().
+        ehi::EventDataDescriptor::from_value(&PROTOCOL_FIELD_VALUE), // protocol value 0 for protobuf
+        ehi::EventDataDescriptor::from_slice(PROTOBUF_VERSION),      // protobuf definition version
         ehi::EventDataDescriptor::from_value(&buffer_rel_loc), // rel_loc for the buffer field.
-        ehi::EventDataDescriptor::from_slice(buffer), // buffer field.
+        ehi::EventDataDescriptor::from_slice(buffer),          // buffer field.
     ])
 }