…tp.Client requests

When enabled via WithClientTrace(true), the integration captures detailed timing
information for DNS lookups, connection establishment, TLS handshakes, and other
HTTP request events as span tags. This provides valuable insights for debugging
network performance issues and connection problems.

Added span tags:
- http.dns.duration_ms: DNS resolution timing
- http.connect.duration_ms: TCP connection establishment timing
- http.tls.duration_ms: TLS handshake timing
- http.get_conn.duration_ms: Connection pool acquisition timing
- http.first_byte.duration_ms: Time to first response byte
- http.connect.error: Connection failure details (when errors occur)
- http.tls.error: TLS handshake failure details (when errors occur)

This enables developers to:
- Distinguish between DNS vs TLS vs connection bottlenecks
- Verify connection pooling is working as expected
- Debug slow network requests with precise timing breakdowns
- Identify specific failure points in the HTTP request lifecycle

The feature is opt-in (disabled by default) to avoid performance overhead
for users who don't need detailed timing information.

Fixes DataDog#1598

Signed-off-by: Jongwon Youn <[email protected]>

As requested in PR review, renamed the function from WithClientTrace to
WithClientTimings to better reflect that this option adds timing information
tags rather than just enabling tracing.