xds: pre-parse custom metric names in WRR load balancer

[sauravzg]

Introduce ParsedMetricName in MetricReportUtils to pre-parse configured custom metric names into Enums and key Strings on config initialization in WeightedRoundRobinLoadBalancerConfig, avoiding String parsing operations in the data path.

This has been done by a combination of a few things

• Streams -> loop
• OptionalDouble -> double
• Pre parsing instead of hot path substring

OrcaReportListener now utilizes pre-parsed ParsedMetricName objects during getCustomMetricUtilization to prevent OptionalDouble heap allocations on the hot path.

Updated test coverage in MetricReportUtilsTest and WeightedRoundRobinLoadBalancerTest.

JMH Benchmark Report: MetricReportUtils Optimization

We performed a benchmark comparison of four different custom metric resolution implementations in the Weighted Round Robin (WRR) load balancer.

Benchmark Results

Benchmark Variant	Average Latency	Normalized Heap Allocations	Speedup
Baseline (String + Streams)	174.46 ns/op	704.00 B/op	1x
ParsedMetricName + Streams	148.95 ns/op	608.00 B/op	~1.1x
String + Loop	81.61 ns/op	240.00 B/op	~2.1x
ParsedMetricName + Loop	52.92 ns/op	144.00 B/op	~3.2x
ParsedMetricName + Unboxed Loop	43.76 ns/op	≈ 0.00 B/op	~4.0x

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[sauravzg]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the custom metric handling in the WeightedRoundRobinLoadBalancer to improve performance. It introduces a ParsedMetricName class and a MetricType enum to pre-parse metric name strings into structured objects during configuration. This change eliminates the need for repeated string parsing and prefix checking whenever a load report is processed. The MetricReportUtils utility was updated to use these parsed types, and the load balancer's internal listeners and pickers were adjusted accordingly. I have no feedback to provide.

[sauravzg]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the WeightedRoundRobinLoadBalancer to use pre-parsed metric names, improving efficiency by avoiding repeated string parsing during load report processing. It introduces a ParsedMetricName class and updates MetricReportUtils to handle metric lookups using these typed objects. Feedback was provided to improve the robustness of the parsing utility by explicitly handling null input.

[sauravzg]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the custom metric handling in the WeightedRoundRobinLoadBalancer by pre-parsing metric names. It introduces a ParsedMetricName class and a MetricType enum in MetricReportUtils to replace string-based lookups with a more efficient typed approach. The WeightedRoundRobinLoadBalancerConfig now parses and validates metric names upon initialization, ignoring and logging any invalid entries. Corresponding updates were made to the load balancer's picker and ORCA report listener, as well as associated unit tests. I have no feedback to provide as there were no review comments to assess.

[sauravzg]

Update: I forgot that I sent it out and since it's pretty late weekend right now, I'll assign a reviewer once I am back next week.