Fix integration test

The `test_using_charm` integration test seems to send empty metadata and receive an empty response, giving a false-pass result. This can be observed in [CI runs](https://github.com/canonical/mlmd-operator/runs/7923034478?check_suite_focus=true#step:5:480) and locally:

```
$ tox -e integration -- --model=mlmd-test --keep-models
integration installed: asttokens==2.0.8,attrs==22.1.0,backcall==0.2.0,bcrypt==4.0.0,cachetools==5.2.0,certifi==2022.6.15,cffi==1.15.1,chardet==3.0.4,cryptography==37.0.4,decorator==5.1.1,executing==1.0.0,google-auth==2.11.0,idna==2.10,iniconfig==1.1.1,ipdb==0.13.9,ipython==8.4.0,jedi==0.18.1,Jinja2==3.1.2,jsonschema==3.2.0,juju==3.0.1,jujubundlelib==0.5.7,kubernetes==24.2.0,macaroonbakery==1.3.1,MarkupSafe==2.1.1,matplotlib-inline==0.1.6,mypy-extensions==0.4.3,oauthlib==3.2.0,oci-image==1.0.0,ops==1.2.0,packaging==21.3,paramiko==2.11.0,parso==0.8.3,pexpect==4.8.0,pickleshare==0.7.5,pluggy==1.0.0,prompt-toolkit==3.0.30,protobuf==3.20.1,ptyprocess==0.7.0,pure-eval==0.2.2,py==1.11.0,pyasn1==0.4.8,pyasn1-modules==0.2.8,pycparser==2.21,Pygments==2.13.0,pymacaroons==0.13.0,PyNaCl==1.5.0,pyparsing==3.0.9,pyRFC3339==1.1,pyrsistent==0.18.1,pytest==7.1.2,pytest-asyncio==0.19.0,pytest-operator==0.22.0,python-dateutil==2.8.2,pytz==2022.2.1,PyYAML==6.0,requests==2.25.0,requests-oauthlib==1.3.1,rsa==4.9,serialized-data-interface==0.3.5,six==1.16.0,stack-data==0.5.0,theblues==0.5.2,toml==0.10.2,tomli==2.0.1,toposort==1.7,traitlets==5.3.0,typing-extensions==4.3.0,typing-inspect==0.8.0,urllib3==1.26.12,wcwidth==0.2.5,websocket-client==1.4.0,websockets==7.0
integration run-test-pre: PYTHONHASHSEED='3683306377'
integration run-test: commands[0] | pytest -vv --tb native --show-capture=no --log-cli-level=INFO -s --model=mlmd-test --keep-models /home/ubuntu/mlmd-operator/tests/integration
============================================================================ test session starts ============================================================================
platform linux -- Python 3.8.10, pytest-7.1.2, pluggy-1.0.0 -- /home/ubuntu/mlmd-operator/.tox/integration/bin/python
cachedir: .tox/integration/.pytest_cache
rootdir: /home/ubuntu/mlmd-operator
plugins: asyncio-0.19.0, operator-0.22.0
asyncio: mode=strict
collected 2 items

tests/integration/test_charm.py::test_build_and_deploy SKIPPED (unconditional skip)
tests/integration/test_charm.py::test_using_charm
------------------------------------------------------------------------------ live log setup -------------------------------------------------------------------------------
INFO pytest_operator.plugin:plugin.py:653 Connecting to existing model my-controller:mlmd-test on unspecified cloud
------------------------------------------------------------------------------- live log call -------------------------------------------------------------------------------
INFO root:test_charm.py:39 Using temporary directory /home/ubuntu/mlmd-operator/.tox/integration/tmp/pytest/test_using_charm0
INFO root:test_charm.py:40 cwd = /home/ubuntu/mlmd-operator
INFO root:test_charm.py:41 script_abs_path = /home/ubuntu/mlmd-operator/tests/integration/data/interact_with_mlmd.sh
+ MODEL=mlmd-test
+ echo MODEL=mlmd-test
MODEL=mlmd-test
+ wget https://raw.githubusercontent.com/google/ml-metadata/master/ml_metadata/proto/metadata_store.proto
--2022-08-31 09:33:53-- https://raw.githubusercontent.com/google/ml-metadata/master/ml_metadata/proto/metadata_store.proto
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 185.199.108.133, 185.199.109.133, 185.199.110.133, ...
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|185.199.108.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 36972 (36K) [text/plain]
Saving to: ‘metadata_store.proto’

metadata_store.proto 100%[========================================================================================>] 36.11K --.-KB/s in 0s

2022-08-31 09:33:53 (110 MB/s) - ‘metadata_store.proto’ saved [36972/36972]

+ wget https://raw.githubusercontent.com/google/ml-metadata/master/ml_metadata/proto/metadata_store_service.proto
--2022-08-31 09:33:53-- https://raw.githubusercontent.com/google/ml-metadata/master/ml_metadata/proto/metadata_store_service.proto
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 185.199.111.133, 185.199.110.133, 185.199.109.133, ...
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|185.199.111.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 39192 (38K) [text/plain]
Saving to: ‘metadata_store_service.proto’

metadata_store_service.proto 100%[========================================================================================>] 38.27K --.-KB/s in 0s

2022-08-31 09:33:53 (89.3 MB/s) - ‘metadata_store_service.proto’ saved [39192/39192]

+ wget -O- https://github.com/fullstorydev/grpcurl/releases/download/v1.8.0/grpcurl_1.8.0_linux_x86_64.tar.gz
+ tar -xzv
--2022-08-31 09:33:53-- https://github.com/fullstorydev/grpcurl/releases/download/v1.8.0/grpcurl_1.8.0_linux_x86_64.tar.gz
Resolving github.com (github.com)... 140.82.113.4
Connecting to github.com (github.com)|140.82.113.4|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://objects.githubusercontent.com/github-production-release-asset-2e65be/111431261/88be8800-4f53-11eb-94fd-6a7ef1143069?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20220831%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220831T093354Z&X-Amz-Expires=300&X-Amz-Signature=34b32b5c3850e69f87bbdeb08d6ef42aa04ac13e68afcb2337fe13692efddfc6&X-Amz-SignedHeaders=host&actor_id=0&key_id=0&repo_id=111431261&response-content-disposition=attachment%3B%20filename%3Dgrpcurl_1.8.0_linux_x86_64.tar.gz&response-content-type=application%2Foctet-stream [following]
--2022-08-31 09:33:54-- https://objects.githubusercontent.com/github-production-release-asset-2e65be/111431261/88be8800-4f53-11eb-94fd-6a7ef1143069?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20220831%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220831T093354Z&X-Amz-Expires=300&X-Amz-Signature=34b32b5c3850e69f87bbdeb08d6ef42aa04ac13e68afcb2337fe13692efddfc6&X-Amz-SignedHeaders=host&actor_id=0&key_id=0&repo_id=111431261&response-content-disposition=attachment%3B%20filename%3Dgrpcurl_1.8.0_linux_x86_64.tar.gz&response-content-type=application%2Foctet-stream
Resolving objects.githubusercontent.com (objects.githubusercontent.com)... 185.199.110.133, 185.199.108.133, 185.199.109.133, ...
Connecting to objects.githubusercontent.com (objects.githubusercontent.com)|185.199.110.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 4986341 (4.8M) [application/octet-stream]
Saving to: ‘STDOUT’

- 0%[ ] 0 --.-KB/s LICENSE
grpcurl
- 100%[========================================================================================>] 4.75M --.-KB/s in 0.1s

2022-08-31 09:33:54 (35.2 MB/s) - written to stdout [4986341/4986341]

+ mkdir -p ml_metadata/proto/
+ mv metadata_store.proto ml_metadata/proto/
++ kubectl get services/mlmd -n mlmd-test -oyaml
++ yq e .spec.clusterIP -
+ SERVICE=10.152.183.202
+ ./grpcurl -v --proto=metadata_store_service.proto --plaintext 10.152.183.202:8080 ml_metadata.MetadataStoreService/GetArtifacts

Resolved method descriptor:
// Gets all the artifacts.
rpc GetArtifacts ( .ml_metadata.GetArtifactsRequest ) returns ( .ml_metadata.GetArtifactsResponse );

Request metadata to send:
(empty)

Response headers received:
accept-encoding: identity,gzip
content-type: application/grpc
grpc-accept-encoding: identity,deflate,gzip

Response contents:
{

}

Response trailers received:
(empty)
Sent 0 requests and received 1 response
PASSED
----------------------------------------------------------------------------- live log teardown -----------------------------------------------------------------------------
INFO pytest_operator.plugin:plugin.py:768 Model status:

Model Controller Cloud/Region Version SLA Timestamp
mlmd-test my-controller myk8s/localhost 2.9.33 unsupported 09:33:54Z

App Version Status Scale Charm Channel Rev Address Exposed Message
mlmd res:oci-image@e2cb9ce active 1 mlmd stable 10 10.152.183.114 no

Unit Workload Agent Address Ports Message
mlmd/0* active idle 10.1.17.61 8080/TCP
```

Start with tests/integration/test_charm.py and tests/integration/data/interact_with_mlmd.sh, then run tox -e integration -- --model=mlmd-test --keep-models to reproduce test_using_charm. Inspect the grpcurl request and response handling so the test cannot pass with empty metadata and an empty response. Done means the integration test validates a meaningful result and fails when the service returns the currently observed empty response.

Written by the indexing model from the issue text.