Enable selective testing of integrations on main and backport branches

Running tests for all integrations on all branches launches dozens of
jobs. If multiple branches are merged in a short period of time, this
can overload CI.

Author: jsoriano

.ci/Jenkinsfile

@@ -272,15 +272,20 @@ def isPrAffected(integrationName) {
     }
   }
 
-  if (env.BRANCH_NAME == "main") {
-    echo "[${integrationName}] PR is affected: running on main branch"
-    return true
+  def from = "";
+  def to = "";
+  if (env.BRANCH_NAME == 'main' || env.BRANCH_NAME.startsWith('backport-')) {
+    echo "[${integrationName}] PR is affected: running on ${env.BRANCH_NAME} branch"
+    from = "origin/${env.BRANCH_NAME}"
+    // If GIT_PREVIOUS_COMMIT is not available, check with previous commit in the branch, this should
+    // be the same as we are squashing and merging single commits.
+    to = "env.GIT_PREVIOUS_COMMIT?.trim()" ? env.GIT_PREVIOUS_COMMIT : "origin/${env.BRANCH_NAME}^"
+  } else {
+    // Source: https://github.com/elastic/apm-pipeline-library/blob/721115cf0fdb2b5a4e1cb6a576bfbb7035d14485/vars/getGitMatchingGroup.groovy#L40-L41
+    from = env.CHANGE_TARGET?.trim() ? "origin/${env.CHANGE_TARGET}" : "${env.GIT_PREVIOUS_COMMIT?.trim() ? env.GIT_PREVIOUS_COMMIT : env.GIT_BASE_COMMIT}"
+    to = env.GIT_BASE_COMMIT
   }
 
-  // Source: https://github.com/elastic/apm-pipeline-library/blob/721115cf0fdb2b5a4e1cb6a576bfbb7035d14485/vars/getGitMatchingGroup.groovy#L40-L41
-  def from = env.CHANGE_TARGET?.trim() ? "origin/${env.CHANGE_TARGET}" : "${env.GIT_PREVIOUS_COMMIT?.trim() ? env.GIT_PREVIOUS_COMMIT : env.GIT_BASE_COMMIT}"
-  def to = env.GIT_BASE_COMMIT
-
   def r = sh(label: "[${integrationName}] git-diff: check non-package files", script: "git diff --name-only \$(git merge-base ${from} ${to}) ${to} | egrep -v '^(packages/|.github/CODEOWNERS)'", returnStatus: true)
   if (r == 0) {
     echo "[${integrationName}] PR is affected: found non-package files"