Respect --instrumentation_filter for Rust coverage

Previously, all Rust targets were instrumented with
-Cinstrument-coverage when running `bazel coverage`, regardless of
--instrumentation_filter. This differs from how coverage works for
Java and C++ targets, where only targets matching the filter are
instrumented.

Add ctx.coverage_instrumented() check so that Rust coverage respects
the same filter, reducing unnecessary recompilation and keeping
coverage reports focused on the code under test.

Author: Tamas Vajk

rust/private/rustc.bzl

@@ -1220,8 +1220,10 @@ def construct_arguments(
         rustc_flags.add("--extern")
         rustc_flags.add("proc_macro")
 
-    if toolchain.coverage_supported and ctx.configuration.coverage_enabled:
-        # https://doc.rust-lang.org/rustc/instrument-coverage.html
+    # Use Bazel's standard instrumentation filter (--instrumentation_filter)
+    # so that only targets matching the filter get instrumented, consistent
+    # with how coverage works for other languages (Java, C++).
+    if toolchain.coverage_supported and ctx.configuration.coverage_enabled and ctx.coverage_instrumented():
         rustc_flags.add("--codegen=instrument-coverage")
 
     if toolchain._experimental_link_std_dylib: