feat: Add grpc.xds.resource_type label to xDS client metrics

[HailongWen]

This is a quick fix to add the missing grpc.xds.resource_type label in xds_client metrics (b/451687047).

[gemini-code-assist]

Summary of Changes

Hello @HailongWen, 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 addresses a missing grpc.xds.resource_type label in xDS client metrics. By introducing and whitelisting this label, it enhances the observability of xDS client operations, allowing for more granular monitoring and analysis of resource-specific metrics.

Highlights

• New Metric Label Constant: Introduced metricLabelKeyGRPCXDSResourceType as a new constant to represent the xDS resource type label.
• Metric Label Whitelisting: Added the newly defined grpc.xds.resource_type label to the allowedMetricLabels map, enabling its use in xDS client metrics.

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[gemini-code-assist]

Code Review

This pull request correctly adds the grpc.xds.resource_type label to the xDS client metrics. The changes are simple and effective, introducing a new constant for the label and whitelisting it for export. I have one minor suggestion regarding code maintainability.

[gemini-code-assist]

For better maintainability and to avoid potential merge conflicts, it's a good practice to keep the keys in this map literal sorted alphabetically. While the current logical grouping by prefix is reasonable, a consistent alphabetical order is more scalable and less ambiguous as the list of labels grows. Please consider sorting all the entries in the allowedMetricLabels map.