feat: Add Kafka internal metrics collection

[tippmar-nr]

Summary

Adds comprehensive Kafka internal metrics collection to the .NET agent. This enables the New Relic Kafka UI to display critical monitoring data from .NET applications using the Confluent Kafka client (librdkafka).

Key changes:

• Statistics callback integration: Non-invasive statistics collection via librdkafka's SetStatisticsHandler, preserving any customer-configured handlers
• Scheduled drain architecture: Lightweight callback caches latest JSON per client; a single scheduled drain task parses and reports metrics once per harvest interval (not on every callback)
• Gauge metric pipeline: New RecordGaugeMetric API on IAgentExperimental for reporting point-in-time values
• Comprehensive metric coverage: Client-level, broker/node-level, topic-level, and partition-level metrics including:
  • Request/response counters, byte totals, message counts
  • Broker RTT latency averages and connection counts
  • Consumer group rebalance metrics, consumer lag, committed offsets
  • Producer batch size averages, record send totals, idempotent state
• Delta computation for cumulative counters: Tracks previous values per client to report deltas for ever-increasing librdkafka counters
• Allocation-optimized parsing: Reusable dictionaries, struct-based metric values, merged single-pass loops, no LINQ in hot paths

Metric format:

MessageBroker/Kafka/Internal/{type}-metrics/client/{clientId}/{metric-name}
MessageBroker/Kafka/Internal/{type}-node-metrics/node/{nodeId}/client/{clientId}/{metric-name}
MessageBroker/Kafka/Internal/{type}-topic-metrics/topic/{topic}/client/{clientId}/{metric-name}
MessageBroker/Kafka/Internal/{type}-metrics/topic/{topic}/partition/{id}/client/{clientId}/{metric-name}

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.38220% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.90%. Comparing base (29da4dd) to head (644196a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Agent/NewRelic/Agent/Core/Agent.cs	0.00%	3 Missing ⚠️
....Agent.Extensions/Helpers/KafkaStatisticsHelper.cs	98.88%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3494      +/-   ##
==========================================
+ Coverage   81.77%   81.90%   +0.13%     
==========================================
  Files         508      509       +1     
  Lines       34220    34411     +191     
  Branches     4040     4077      +37     
==========================================
+ Hits        27984    28185     +201     
+ Misses       5269     5257      -12     
- Partials      967      969       +2     

Flag	Coverage Δ
Agent	82.90% <97.38%> (+0.13%)	⬆️
Profiler	71.75% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...elic/Agent/Core/AgentHealth/AgentHealthReporter.cs	91.18% <100.00%> (+0.05%)	⬆️
.../NewRelic/Agent/Core/WireModels/MetricWireModel.cs	96.78% <100.00%> (+0.02%)	⬆️
....Agent.Extensions/Helpers/KafkaStatisticsHelper.cs	98.88% <98.88%> (ø)
src/Agent/NewRelic/Agent/Core/Agent.cs	75.07% <0.00%> (-0.64%)	⬇️

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.