Copilot instructions (open-telemetry#15393)

Author: jaydeluca

.github/copilot-instructions.md

@@ -0,0 +1,52 @@
+# OpenTelemetry Java Instrumentation
+
+## Testing
+
+Tests use AssertJ for assertions and JUnit 5 as the testing framework
+
+Test classes and methods should not be public
+
+When registering tests in gradle configurations, if using `val testName by registering(Test::class) {`...
+then you need to include `testClassesDirs` and `classpath` like so:
+
+```
+val testExperimental by registering(Test::class) {
+  testClassesDirs = sourceSets.test.get().output.classesDirs
+  classpath = sourceSets.test.get().runtimeClasspath
+  ...
+}
+```
+
+## General Java guidelines
+
+* Always import classes when possible (i.e. don't use fully qualified class names in code).
+
+## Gradle CLI
+
+Never use the `--rerun-tasks` flag unless explicitly asked to use this option.
+
+Gradle automatically detects changes and re-runs tasks automatically when needed. Using `--rerun-tasks`
+is wasteful and slows down builds unnecessarily.
+
+## Throwing exceptions
+
+When writing instrumentation, you have to be really careful about throwing exceptions. For library
+instrumentations it might be acceptable, but in javaagent code you shouldn't throw exceptions
+(keep in mind that javaagent instrumentations sometimes use library instrumentations).
+
+In javaagent instrumentations we try not to break applications. If there are changes in the instrumented
+library that are not compatible with the instrumentation we disable the instrumentation instead of letting
+it fail. This is handled by muzzle. In javaagent instrumentations you should not fail if the methods
+that you need don't exist.
+
+## Javaagent Instrumentation
+
+### Java8BytecodeBridge
+
+When to use `Java8BytecodeBridge.currentContext()` vs `Context.current()` ?
+
+Using `Context.current()` is preferred. `Java8BytecodeBridge.currentContext()` is for using inside
+advice. We need this method because advice code is inlined in the instrumented method as it is.
+Since `Context.current()` is a static interface method it will cause a bytecode verification error
+when it is inserted into a pre 8 class. `Java8BytecodeBridge.currentContext()` is a regular class
+static method and can be used in any class version.