[Tracing] Provide more subspans

Similar to #868, but represented as spans.

Right now, requests appear to be taking way longer than I would expect, and I suspect that's because of the connection pool warming up, not the actual request. This should be visible in the emitted spans.

From #868:

• How long did this request spend in the connection pool queue
• How long did it take to make a connection (if one didn't already exist)
• How long did it take to send the request
• DNS timings

Mapped to spans, one way this could look that'd allow me to better understand where time is being spent:

| ---------------------------------------------- HTTPClient.execute ----------------------------------------------------- |
| ----------------------- Request 1 ---------------------- | | --------- Request 2 (redirect to the same host) -------- |
| ------ New connection ------ | | ----- HTTP req/res ---- | | --- Queued/reused connection --- | | -- HTTP req/res -- |

And zooming in:

| ------------------------ New connection ------------------------- |
| --- DNS lookup --- | | --- TCP est --- | | --- TLS handshake --- |

And (below is an example of bidi streaming, when the request and response bodies overlap; in a simple non-streaming example, the request and response wouldn't overlap - important bit is that all 4 subspans would have "HTTP req/res" as the parent span)

| ------------------------------------------- HTTP req/res ------------------------------------------ |
| --- HTTP req head --- | | ------------ HTTP req body ------ |
                                           | --- HTTP res head --- | | ------- HTTP res body ---- |

The goal is for you to be able to gland at the trace and quickly identify where the bulk of time was spent - mainly: pre-request or inside the request.

Start by reading issue #868 and tracing around HTTPClient.execute, then identify how connection pooling, DNS, connection setup, and request/response streaming are currently represented in emitted spans. Done means the trace exposes the requested nested timing stages, including queueing or reuse, connection establishment, DNS, and HTTP request/response activity.