[pkg/ottl] Add e2e tests (open-telemetry#30152)

**Description:** 
Adds e2e tests for OTTL functionality and all `ottlfuncs` functions.

Includes the bug fix from
open-telemetry#30151

**Link to tracking Issue:** <Issue number if applicable>
Closes
open-telemetry#28642

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>
Updated contributing doc

pkg/ottl/CONTRIBUTING.md

@@ -19,7 +19,7 @@ When adding new values to the grammar you must:
 
 All new functions must be added via a new file. Function files must start with `func_`. Functions must be placed in `ottlfuncs`.
 
-Unit tests must be added for all new functions. Unit test files must start with `func_` and end in `_test`. Unit tests must be placed in the same directory as the function. Functions that are not specific to a pipeline should be tested independently of any specific pipeline. Functions that are specific to a pipeline should be tests against that pipeline.
+Unit tests must be added for all new functions. Unit test files must start with `func_` and end in `_test`. Unit tests must be placed in the same directory as the function. Functions that are not specific to a pipeline should be tested independently of any specific pipeline. Functions that are specific to a pipeline should be tests against that pipeline. End-to-end tests must be added in the `e2e` directory.
 
 Function names should follow the [Function Syntax Guidelines](ottlfuncs/README.md#function-syntax)