Problem

The first time Ustr::from() is called, it takes 10+ ms to init. Right now this is on the critical path and adds to cold start time.

This PR

Start it early in a separate thread, so when it is needed later, it will be already initialized.

Result

Using extensiononly stack on Node 22:

Before

• Datadog Next-Gen Extension ready in: 54.1 ±2.2 ms (95% confidence interval, 21 data points)
• Metrics aggregator created in: 10+ ms (10,000+ us)

After

• Datadog Next-Gen Extension ready in: 49.7±1.9 ms (95% confidence level, 10 data points)
• Metrics aggregator created in: 277±186 us (95% confidence level, 10 data points)

Thanks @duncanista for identifying this performance bottleneck!
Jira: https://datadoghq.atlassian.net/browse/SVLS-7456