Feature: support custom key prefix for issue 957

[uniqueni]

Proposed Solution

Add MC_STORE_ETCD_KEY_PREFIX and MC_METADATA_ETCD_KEY_PREFIX environment variable to customize the key prefix

Use Cases

• Multi-environment: Share etcd between dev/test/prod
• Multi-cluster: Run multiple clusters on same infrastructure
• Multi-tenancy: Isolate metadata per tenant/application

[gemini-code-assist]

Summary of Changes

Hello @uniqueni, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new feature that allows users to customize the metadata key prefix for etcd via the MC_METADATA_KEY_PREFIX environment variable. This change enables better isolation and organization of metadata in multi-environment, multi-cluster, and multi-tenancy deployments without requiring any code changes or affecting other storage backends.

Highlights

• New Feature: Introduces the MC_METADATA_KEY_PREFIX environment variable to allow customization of the metadata key prefix specifically for etcd.
• Use Cases: Supports multi-environment, multi-cluster, and multi-tenancy scenarios by isolating metadata.
• Backward Compatibility: Maintains backward compatibility by defaulting to the current behavior if the environment variable is not set.
• Implementation Details: Modifies the TransferMetadata class to read the MC_METADATA_KEY_PREFIX environment variable and construct dynamic prefixes for metadata keys.

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[gemini-code-assist]

Code Review

This pull request introduces a useful feature to support custom metadata key prefixes for etcd, which will help in multi-tenant or multi-environment setups. The implementation is straightforward, adding the prefix logic within the TransferMetadata class and isolating it to the etcd backend.

The code changes are mostly correct, but I've found one issue regarding protocol detection that could cause the feature to not work with certain valid connection strings. I've left a specific comment with a suggestion to fix it.

Additionally, this new functionality is not covered by any tests. It would be beneficial to add unit tests to transfer_metadata_test.cpp to verify:

• The custom prefix is correctly applied when MC_METADATA_KEY_PREFIX is set for an etcd connection.
• The default prefix is used when the environment variable is not set.
• The environment variable is ignored for non-etcd backends (like redis or http).

Adding these tests will ensure the feature works as expected and prevent future regressions.