ci(java): restrict presubmit samples ITs to only snapshot

This is to reduce resource consumption since we often times hit resource quota limit in samples testing.
e.g. googleapis/java-bigquerydatatransfer#410

Source-Author: Stephanie Wang <stephaniewang526@users.noreply.github.com>
Source-Date: Thu Oct 15 17:16:14 2020 -0400
Source-Repo: googleapis/synthtool
Source-Sha: 27e0e916cbfdb3d5ff6639b686cc04f78a0b0386
Source-Link: googleapis/synthtool@27e0e91

.kokoro/build.sh

@@ -69,9 +69,16 @@ integration)
     RETURN_CODE=$?
     ;;
 samples)
-    if [[ -f samples/pom.xml ]]
+    SAMPLES_DIR=samples
+    # only run ITs in snapshot/ on presubmit PRs. run ITs in all 3 samples/ subdirectories otherwise.
+    if [[ ! -z ${KOKORO_GITHUB_PULL_REQUEST_NUMBER} ]]
     then
-        pushd samples
+      SAMPLES_DIR=samples/snapshot
+    fi
+
+    if [[ -f ${SAMPLES_DIR}/pom.xml ]]
+    then
+        pushd {SAMPLES_DIR}
         mvn -B \
           -Penable-samples \
           -DtrimStackTrace=false \

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-cloud-bom.git",
-        "sha": "fe23054b7fa800eaa2479ac03b6806d33c4b7ffe"
+        "sha": "21a8020c2f77aadf7469c838cc5717e26d167ccc"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "b65ef07d99946d23e900ef2cc490274a16edd336"
+        "sha": "27e0e916cbfdb3d5ff6639b686cc04f78a0b0386"
       }
     }
   ],