[SLES-2499] feat: Support sending trace stats via http proxy

[lym953]

Overview

Support sending trace stats to Datadog when the Lambda is in a VPC with a proxy.

Testing

Steps

1. Set up a Lambda in a VPC with a proxy. See Using an agent proxy (internal)
2. Build an extension layer and install it on the Lambda
3. Invoke the Lambda. After 20 seconds, invoke it again. This should force trace stats for the first invocation to be flushed.

Result

Before: stats flush failed with error:

Error sending stats: Failed to send trace stats: client error (Connect)

After: stats flush was successful.

Dependency

DataDog/libdatadog#1345

Notes

GitHub issue: #860
Jira: https://datadoghq.atlassian.net/browse/SLES-2499