Adds API spec coverage report.

.github/workflows/coverage.yml

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+name: API Coverage
+
+on: [push, pull_request]
+
+env:
+  JAVA_VERSION: 11
+  OPENSEARCH_INITIAL_ADMIN_PASSWORD: BobgG7YrtsdKf9M
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@v2
+      - name: Build and Run Docker Container
+        run: |
+          docker build coverage --tag opensearch-with-api-plugin
+          docker run -d -p 9200:9200 -p 9600:9600 -e "discovery.type=single-node" -e OPENSEARCH_INITIAL_ADMIN_PASSWORD="$OPENSEARCH_INITIAL_ADMIN_PASSWORD" opensearch-with-api-plugin
+          sleep 15
+      - name: Display OpenSearch Info
+        run: |
+          curl   -ks -u "admin:$OPENSEARCH_INITIAL_ADMIN_PASSWORD" https://localhost:9200/ | jq
+      - name: Dump and Compare API
+        run: |
+          curl -ks -u "admin:$OPENSEARCH_INITIAL_ADMIN_PASSWORD" https://localhost:9200/_plugins/api | jq > OpenSearch.auto.openapi.json
+          docker run --rm --mount type=bind,source=.,target=/specs openapitools/openapi-diff:latest /specs/OpenSearch.openapi.json /specs/OpenSearch.auto.openapi.json --json /specs/diff.json
+      - name: Show Diff
+        run: |
+          echo "-------- Missing APIs"
+          jq -r '.newEndpoints | group_by(.pathUrl)[] | "\(.[0].pathUrl): \([.[].method])"' diff.json
+          echo "-------- Legacy APIs"
+          jq -r '.missingEndpoints | group_by(.pathUrl)[] | "\(.[0].pathUrl): \([.[].method])"' diff.json
+      - name: Gather Coverage
+        id: coverage
+        shell: bash
+        run: |
+          current=`jq -r '.paths | keys | length' OpenSearch.openapi.json`
+          total=`jq -r '.paths | keys | length' OpenSearch.auto.openapi.json`
+          percent=$((current * 100 / total))
+          echo "API specs implemented for $current/$total ($percent%) APIs."

coverage/Dockerfile

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+ARG OPENSEARCH_VERSION=2.12.0
+FROM opensearchproject/opensearch:${OPENSEARCH_VERSION}
+ARG OPENSEARCH_VERSION
+RUN /usr/share/opensearch/bin/opensearch-plugin \
+    install \
+    --batch \
+    https://github.com/dblock/opensearch-api/releases/download/v${OPENSEARCH_VERSION}/opensearch-api-${OPENSEARCH_VERSION}.0.zip

coverage/README.md

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+### API Coverage
+
+Uses the [opensearch-api plugin](https://github.com/dblock/opensearch-api), and [openapi-diff](https://github.com/OpenAPITools/openapi-diff) to show the difference between OpenSearch APIs, and the [OpenAPI spec checked into this repo](../OpenSearch.openapi.json).
+
+API coverage is run on all pull requests via the [coverage workflow](../.github/workflows/coverage.yml).
+
+