Fix ListArtifacts and get-artifact compatibility errors with proxied artifacts

[dbczumar]

What changes are proposed in this pull request?

Fix ListArtifacts and get-artifact compatibility errors with proxied artifacts. Addresses #5377.

How is this patch tested?

• Unit tests
• Manual tests conducted via the MLflow UI (artifact listing and downloading from the artifact viewer for HTTP, mlflow-artifacts, and non-proxied - e.g. /tmp/mylocaldirectory routes)

Does this PR change the documentation?

• No. You can skip the rest of this section.
• Yes. Make sure the changed pages / sections render correctly by following the steps below.

1. Check the status of the ci/circleci: build_doc check. If it's successful, proceed to the
   next step, otherwise fix it.
2. Click Details on the right to open the job page of CircleCI.
3. Click the Artifacts tab.
4. Click docs/build/html/index.html.
5. Find the changed pages / sections and make sure they render correctly.

Release Notes

Fix listing and downloading of artifacts from the MLflow UI when the MLflow server is operating in --serve-artifacts mode.

Is this a user-facing change?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release notes for MLflow users.

What component(s), interfaces, languages, and integrations does this PR affect?

Components

• area/artifacts: Artifact stores and artifact logging
• area/build: Build and test infrastructure for MLflow
• area/docs: MLflow documentation pages
• area/examples: Example code
• area/model-registry: Model Registry service, APIs, and the fluent client calls for Model Registry
• area/models: MLmodel format, model serialization/deserialization, flavors
• area/projects: MLproject format, project running backends
• area/scoring: MLflow Model server, model deployment tools, Spark UDFs
• area/server-infra: MLflow Tracking server backend
• area/tracking: Tracking Service, tracking client APIs, autologging

Interface

• area/uiux: Front-end, user experience, plotting, JavaScript, JavaScript dev server
• area/docker: Docker use across MLflow's components, such as MLflow Projects and MLflow Models
• area/sqlalchemy: Use of SQLAlchemy in the Tracking Service or Model Registry
• area/windows: Windows support

Language

• language/r: R APIs and clients
• language/java: Java APIs and clients
• language/new: Proposals for new client languages

Integrations

• integrations/azure: Azure and Azure ML integrations
• integrations/sagemaker: SageMaker integrations
• integrations/databricks: Databricks integrations

How should the PR be classified in the release notes? Choose one:

• rn/breaking-change - The PR will be mentioned in the "Breaking Changes" section
• rn/none - No description will be included. The PR will be mentioned only by the PR number in the "Small Bugfixes and Documentation Updates" section
• rn/feature - A new user-facing feature worth mentioning in the release notes
• rn/bug-fix - A user-facing bug fix worth mentioning in the release notes
• rn/documentation - A user-facing documentation change worth mentioning in the release notes

[dbczumar]

autoformat

[eedeleon]

Given the MLflow Tracking URI tends to be the base for the artifact URI(url is passed through here for that reason)

Is there a way to deploy a proxied artifact server with artifact uri mlflow-artifacts, that gets read client side into
Tracking URI + /api/2.0/mlflow/artifacts

Effectively, can this short cut you did for testing be a supported feature in the client + mlflow server call for proxied artifacts?

[dbczumar]

Hi Eddie! Thanks for asking about this - we considered using api/2.0/mlflow/artifacts as the base but opted to go with mlflow-artifacts to distinguish the API surface for proxied artifact uploads from the API surface for MLflow Tracking. If we included these in the MLflow Tracking surface, all implementers of MLflow Tracking would be required to support it.

[dbczumar]

autoformat

[BenWilson2]

Looks great! The docstrings are amazingly clear and will help out substantially if we ever need to touch this logic again.