auto-instrument the std network clients when observability is on

[kacy]

summary

phase 4 of the observability sprint: when tracing is active (after obs.init), the std http and grpc clients now create a client span, inject a W3C traceparent into the outbound request, and record RED metrics — with no span code in the app. a small pair of server-side helpers lets a handler record a server span in two lines.

every hook gates on trace.is_active() first, so the default (uninstrumented) program pays a single bool check and behaves byte-identically. no compiler change, no bootstrap-seed refresh — std only.

hooks

• grpc Conn.unary — CLIENT span named after the full method, rpc.system/rpc.method attrs, traceparent injected into the request headers, span status from the grpc-status, and rpc_client_requests_total{rpc_method,grpc_status} + rpc_client_duration_ms{rpc_method}. the streaming opens propagate the current trace context via traceparent too.
• http ClientRequest.send / send_tls_with_config — CLIENT span "HTTP <method>", http.request.method/http.url attrs, traceparent header, status from the response code (>= 400 or a transport error is an error), and http_client_requests_total{method,status} + http_client_duration_ms{method}.
• http begin_server_span(req) / end_server_span(span, status) — new public helpers. begin extracts the inbound traceparent so the SERVER span joins the caller's trace; end sets status, ends, and records http_server_requests_total{method,route,status} + http_server_duration_ms{method,route}. usage is documented in the doc comment (two lines around a handler, since there is no server framework).

what was tested

• pith test on std/net/grpc.pith (24), std/net/http.pith (23), std/net/http2/connection.pith (36) — all pass.
• make run-examples (86 passed) and make run-regressions (218 passed) — proves off-by-default is transparent.
• an end-to-end program: a traced http client call to a traced http server sharing one connection. drained spans confirmed the client and server spans share the same trace_id and the server span's parent_id equals the client span's span_id (traceparent propagated across the wire), and metrics.snapshot_text() showed both the client and server RED counters.

design notes

• the hooks are additive wrapping: no existing signature or return value changed. send/send_tls_with_config/unary return exactly what they did.
• db-driver spans were left as a follow-up to keep the change small and focused on the network clients.