Run Rust CI only if rust directory is modified (#1529)

Avoid running CI checks unnecessarily. Need to validate if this causes
any issue with "required check". Hopefully not, as this is a pattern
used in other OTel repos too.

Later to be expanded to Go also.

Author: cijothomas

.github/workflows/rust-ci.yml

@@ -20,7 +20,25 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  check-changes:
+    runs-on: ubuntu-latest
+    outputs:
+      rust: ${{ steps.filter.outputs.rust }}
+    steps:
+      - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
+        with:
+          submodules: true
+      - uses: dorny/paths-filter@v2
+        id: filter
+        with:
+          filters: |
+            rust:
+              - 'rust/**'
+              - '.github/workflows/rust-ci.yml'
+
   test_and_coverage:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -56,6 +74,8 @@ jobs:
           fail_ci_if_error: true
 
   fmt:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -80,6 +100,8 @@ jobs:
         working-directory: ./rust/${{ matrix.folder }}
 
   clippy:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -105,6 +127,8 @@ jobs:
         working-directory: ./rust/${{ matrix.folder }}
 
   deny:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -149,6 +173,8 @@ jobs:
           manifest-path: ./rust/${{ matrix.folder }}/Cargo.toml
 
   docs:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -172,6 +198,8 @@ jobs:
         working-directory: ./rust/${{ matrix.folder }}
 
   structure_check:
+    needs: check-changes
+    if: needs.check-changes.outputs.rust == 'true'
     strategy:
       fail-fast: false
       matrix: