ci: correctly identify base branch for microbenchmarks (#13195)

This PR addresses two separate issues with the microbenchmarks:

1. The `check-big-regressions` job for the microbenchmarks has been
failing on some release branches due to the following silent error in
the `baseline:detect` job:
```
none of the git remotes configured for this repository point to a known GitHub host. To tell gh about a new GitHub host, please use `gh auth login`
```
See example
[here](https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/895493368).
As a result, the subsequent `git describe --tags` command would always
be comparing the release branch to the last tag on `main`, not on the
release branch. Adding the `git config`s and `git remote set-url origin`
in the `microbenchmarks.yml` file remediates this.

2. Given that we're moving forward with NO default backporting (except
for 2.21), there will be more cases where a release is tagged on the
`main` branch, not on its own minor release branch like `3.5`. This is
updated in the `detect-baseline.sh` file to reflect that additional
condition. Now if there is no minor release branch, it defaults to main
(see example
[here](https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/896300765)).

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: erikayasuda

.gitlab/benchmarks/microbenchmarks.yml

@@ -64,16 +64,18 @@ baseline:detect:
   variables:
     UPSTREAM_BRANCH: $CI_COMMIT_REF_NAME
   script: |
+    GITHUB_REPO_URL="https://github.com/DataDog/dd-trace-py.git"
+    git config --global --add safe.directory "${GITHUB_REPO_URL}"
+    git config --global --add safe.directory ${CI_PROJECT_DIR}
+    git remote set-url origin "${GITHUB_REPO_URL}"
+
     if [ -z ${GH_TOKEN} ]
     then
       aws ssm get-parameter --region us-east-1 --name ci.$CI_PROJECT_NAME.gh_token --with-decryption --query "Parameter.Value" --out text > token
       gh auth login --with-token < token
       rm token
     fi
 
-    # Needed to run `git describe`
-    git config --global --add safe.directory ${CI_PROJECT_DIR}
-
     # Determine baseline to test against and save env variables into `baseline.env`
     .gitlab/benchmarks/steps/detect-baseline.sh
   artifacts:

.gitlab/benchmarks/steps/detect-baseline.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-set -e -o pipefail
+set -ex -o pipefail
 
 # Script to determine the baseline version to compare against for a given CI run
 # The results are written as environment variables to a `baseline.env` file
@@ -25,10 +25,19 @@ if [ "${UPSTREAM_BRANCH}" == "main" ]; then
 elif [[ "${UPSTREAM_BRANCH}" =~ ^v[0-9]+\.[0-9]+\.[0-9]+ ]]; then
   # Baseline branch is the major.minor version of the tag
   BASELINE_BRANCH=$(echo "${UPSTREAM_BRANCH:1}" | cut -d. -f1-2)
-  echo "BASELINE_BRANCH=${BASELINE_BRANCH}" | tee baseline.env
+
+  # Check if a release branch exists or not
+  if git ls-remote --exit-code --heads origin "${BASELINE_BRANCH}" > /dev/null; then
+    echo "Found remote branch origin/${BASELINE_BRANCH}"
+  else
+    echo "Remote branch origin/${BASELINE_BRANCH} not found. Falling back to main."
+    BASELINE_BRANCH="main"
+  fi
   # Don't forget to omit the current tag we are testing
   BASELINE_TAG=$(git describe --tags --abbrev=0 --exclude "*rc*" --exclude "${UPSTREAM_BRANCH}" "origin/${BASELINE_BRANCH}" || echo "")
 
+  echo "BASELINE_BRANCH=${BASELINE_BRANCH}" | tee baseline.env
+
 # If this is a release branch (e.g. `2.21`) then test against the latest version from that point (e.g. v2.21.2 or v2.20.x)
 elif [[ "${UPSTREAM_BRANCH}" =~ ^[0-9]+\.[0-9]+$ ]]; then
   BASELINE_BRANCH="${UPSTREAM_BRANCH}"