Add synchronous mode for serverless environments

[duncanista]

Summary

• Add StatsdConfig.SynchronousMode option that bypasses the async background worker and sends metrics directly on the calling thread
• Extract IStatsBufferize interface from StatsBufferize to enable swapping async/sync implementations
• Suppress the telemetry background timer when synchronous mode is enabled

Motivation

In AWS Lambda and similar serverless environments, the runtime freezes the sandbox immediately after the handler returns. The DogStatsD client processes metrics on a background thread via AsynchronousWorker, so buffered metrics can be lost when the freeze hits. The existing Flush() method has a hardcoded 3-second timeout and still relies on a background thread to drain the queue — making it fundamentally unreliable in serverless contexts.

SynchronousMode eliminates this class of bugs entirely: no background threads, no async queue, no race with the sandbox freeze. Metrics are routed and serialized on the calling thread, batched into efficient UDP packets via the existing BufferBuilder, and the user calls Flush() at the end of their handler to send any remaining partial buffer.

Usage

var dogstatsd = new DogStatsdService();
dogstatsd.Configure(new StatsdConfig
{
    StatsdServerName = "127.0.0.1",
    SynchronousMode = true,
});

// In Lambda handler:
dogstatsd.Counter("lambda.invocations", 1);
dogstatsd.Flush(); // all metrics sent before handler returns

Test plan

• All 384 existing tests pass (pure refactoring in Phase 1)
• 7 unit tests for SynchronousSender (pool behavior, send/flush, batching, auto-flush on full buffer, dispose triggers flush, thread safety, empty flush)
• 2 builder tests verifying sync mode wiring (skips async CreateStatsBufferize, passes synchronousMode to telemetry)
• 10 integration tests with real UDP listener (counter, gauge, histogram, distribution, timer, set, tags, event, service check, dispose-flush)
• Manual validation in a Lambda environment

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