Add additional optional metadata fields (#186)

* Add additional optional metadata fields

* Add comment about namespace non-global-uniqueness

* Remove workspace for now

build/bazel/remote/execution/v2/remote_execution.proto

@@ -1690,4 +1690,18 @@ message RequestMetadata {
   // An identifier to tie multiple tool invocations together. For example,
   // runs of foo_test, bar_test and baz_test on a post-submit of a given patch.
   string correlated_invocations_id = 4;
+
+  // A brief description of the kind of action, for example, CppCompile or GoLink.
+  // There is no standard agreed set of values for this, and they are expected to vary between different client tools.
+  string action_mnemonic = 5;
+
+  // An identifier for the target which produced this action.
+  // No guarantees are made around how many actions may relate to a single target.
+  string target_id = 6;
+
+  // An identifier for the configuration in which the target was built,
+  // e.g. for differentiating building host tools or different target platforms.
+  // There is no expectation that this value will have any particular structure,
+  // or equality across invocations, though some client tools may offer these guarantees.
+  string configuration_id = 7;
 }