This script parses the value from the code coverage report file and is able to update the secrets value 'CODE_COVERAGE'.

Get_and_update_coverage.sh

# Get code coverage generated by Jacoco reports
chmod +x $BITRISE_SOURCE_DIR/build/reports/jacoco/index.html
CURRENT_COVERAGE=$(cat $BITRISE_SOURCE_DIR/build/reports/jacoco/index.html | grep -o 'Total[^%]*%' | sed 's/<.*>/ /; s/Total//')
echo ${CURRENT_COVERAGE//[[:blank:]]/}

# Try delete previous value in Bitrise Secrets
curl -X DELETE 'https://api.bitrise.io/v0.1/apps/$APP_ID/secrets/CODE_COVERAGE' \
-H 'Content-Type: application/json' \
-H 'Authorization: $BITRISE_PERSONAL_ACCESS_TOKEN'> delete_secret_result.json

# Update Code Coverage in Bitrise Secrets
curl -X PUT 'https://api.bitrise.io/v0.1/apps/$APP_ID/secrets/CODE_COVERAGE' \
-H "accept: application/json" \
-H "Authorization: $BITRISE_PERSONAL_ACCESS_TOKEN" \
-H "Content-Type: application/json" \
-d "{\"expand_in_step_inputs\": false,  \"is_exposed_for_pull_requests\": true, \"is_protected\": false, \"value\": \"$CURRENT_COVERAGE\"}"  > secret-result.json