[CI] improvement of code coverage check (grafana#7428)

**What this PR does / why we need it**:
changed the check step to compare code coverage with `target` branch,
not with `main`.

Previously, it compared code coverage with coverage in the `main`
branch. to do this, Drone checked out `main` branch and tried to run the
tests to report the coverage. Also, Drone ran the tests on the current
branch, and after that compared the results and reported them to PR as a
comment.

However, if the PR is opened against not `main` branch, for example,
against `release-2.6.x` branch, drone tried to compare coverage between
the source branch from PR and `main`. However, `main` might contain
breaking changes that prevent Drone from compiling the sources or from
running the tests successfully.

Last time, it was Go version. Loki in `main` branch was migrated to
go1.19 and `loki-build-image` was updated. however, Drone CI on release
branch ran the pipeline on old `loki-build-image` that was not able to
compile the Loki from `main` branch.

.drone/drone.jsonnet

@@ -518,23 +518,28 @@ local manifest_ecr(apps, archs) = pipeline('manifest-ecr') {
     steps: [
       make('check-drone-drift', container=false) { depends_on: ['clone'] },
       make('check-generated-files', container=false) { depends_on: ['clone'] },
-      run('clone-main', commands=['cd ..', 'git clone $CI_REPO_REMOTE loki-main', 'cd -']) { depends_on: ['clone'] },
-      make('test', container=false) { depends_on: ['clone', 'clone-main'] },
-      run('test-main', commands=['cd ../loki-main', 'BUILD_IN_CONTAINER=false make test']) { depends_on: ['clone-main'] },
+      run('clone-target-branch', commands=[
+        'cd ..',
+        'echo "cloning "$DRONE_TARGET_BRANCH ',
+        'git clone -b $DRONE_TARGET_BRANCH $CI_REPO_REMOTE loki-target-branch',
+        'cd -',
+      ]) { depends_on: ['clone'], when: onPRs },
+      make('test', container=false) { depends_on: ['clone', 'clone-target-branch'] },
+      run('test-target-branch', commands=['cd ../loki-target-branch', 'BUILD_IN_CONTAINER=false make test']) { depends_on: ['clone-target-branch'], when: onPRs },
       make('compare-coverage', container=false, args=[
-        'old=../loki-main/test_results.txt',
+        'old=../loki-target-branch/test_results.txt',
         'new=test_results.txt',
         'packages=ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki',
         '> diff.txt',
-      ]) { depends_on: ['test', 'test-main'] },
+      ]) { depends_on: ['test', 'test-target-branch'], when: onPRs },
       run('report-coverage', commands=[
         "pull=$(echo $CI_COMMIT_REF | awk -F '/' '{print $3}')",
         "body=$(jq -Rs '{body: . }' diff.txt)",
         'curl -X POST -u $USER:$TOKEN -H "Accept: application/vnd.github.v3+json" https://api.github.com/repos/grafana/loki/issues/$pull/comments -d "$body" > /dev/null',
       ], env={
         USER: 'grafanabot',
         TOKEN: { from_secret: github_secret.name },
-      }) { depends_on: ['compare-coverage'] },
+      }) { depends_on: ['compare-coverage'], when: onPRs },
       make('lint', container=false) { depends_on: ['clone', 'check-generated-files'] },
       make('check-mod', container=false) { depends_on: ['clone', 'test', 'lint'] },
       {

.drone/drone.yml

@@ -104,39 +104,49 @@ steps:
   name: check-generated-files
 - commands:
   - cd ..
-  - git clone $CI_REPO_REMOTE loki-main
+  - 'echo "cloning "$DRONE_TARGET_BRANCH '
+  - git clone -b $DRONE_TARGET_BRANCH $CI_REPO_REMOTE loki-target-branch
   - cd -
   depends_on:
   - clone
   environment: {}
   image: grafana/loki-build-image:0.24.2
-  name: clone-main
+  name: clone-target-branch
+  when:
+    event:
+    - pull_request
 - commands:
   - make BUILD_IN_CONTAINER=false test
   depends_on:
   - clone
-  - clone-main
+  - clone-target-branch
   environment: {}
   image: grafana/loki-build-image:0.24.2
   name: test
 - commands:
-  - cd ../loki-main
+  - cd ../loki-target-branch
   - BUILD_IN_CONTAINER=false make test
   depends_on:
-  - clone-main
+  - clone-target-branch
   environment: {}
   image: grafana/loki-build-image:0.24.2
-  name: test-main
+  name: test-target-branch
+  when:
+    event:
+    - pull_request
 - commands:
-  - make BUILD_IN_CONTAINER=false compare-coverage old=../loki-main/test_results.txt
+  - make BUILD_IN_CONTAINER=false compare-coverage old=../loki-target-branch/test_results.txt
     new=test_results.txt packages=ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki
     > diff.txt
   depends_on:
   - test
-  - test-main
+  - test-target-branch
   environment: {}
   image: grafana/loki-build-image:0.24.2
   name: compare-coverage
+  when:
+    event:
+    - pull_request
 - commands:
   - pull=$(echo $CI_COMMIT_REF | awk -F '/' '{print $3}')
   - 'body=$(jq -Rs ''{body: . }'' diff.txt)'
@@ -150,6 +160,9 @@ steps:
     USER: grafanabot
   image: grafana/loki-build-image:0.24.2
   name: report-coverage
+  when:
+    event:
+    - pull_request
 - commands:
   - make BUILD_IN_CONTAINER=false lint
   depends_on:
@@ -1597,6 +1610,6 @@ kind: secret
 name: gpg_private_key
 ---
 kind: signature
-hmac: 63a2da744f6d908e4588aba33231bbe438a6c2438bd1f288af60404b137d5df7
+hmac: 7103f179cdc3517cf3ddb2d31c3ead90a400dd5416558ef4cccf5d54063cc923
 
 ...