Log parameters of SledAgent POST requests

Currently, SledAgent does not log parameters of POST requests, only occasionally logging the request parameters of if it results in a failure:

2026-04-03 18:17:14.152Z INFO SledAgent/656 (dropshot (SledAgent)) on BRM44220009: request completed
    error_message_external = Service Unavailable
    error_message_internal = Failed to ensure volume 'oxp_a5a52f47-9c9a-4519-83dc-abc56619495d/local_storage_unencrypted/1d843bb7-4ec7-4ff4-adb7-f82d39f9fe85/vol': created but not ready yet: still zero initializing
    file = /home/build/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/dropshot-0.16.7/src/server.rs:855
    latency_us = 1776444
    local_addr = [fd00:1122:3344:11d::1]:12345
    method = POST
    remote_addr = [fd00:1122:3344:11d::22]:38647
    req_id = 036ec691-1cfa-451e-a81d-63f84b6f28b0
    response_code = 503
    uri = /local-storage

Successful request shows no parameters:

2026-04-03 18:10:14.689Z INFO SledAgent/656 (dropshot (SledAgent)) on BRM44220009: request completed
    file = /home/build/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/dropshot-0.16.7/src/server.rs:867
    latency_us = 110199
    local_addr = [fd00:1122:3344:11d::1]:12345
    method = POST
    remote_addr = [fd00:1122:3344:11d::22]:58440
    req_id = a523f7ea-7b17-49b8-816d-aa77f7ffe6f5
    response_code = 204
    uri = /local-storage

We need a way to correlate successful requests against Nexus, which also does not log requests parameters. This makes tracing distributed operations difficult.

Start by locating SledAgent's request-completion logging and POST endpoint handlers, then compare the successful and failed request records shown in the issue. Check how the Dropshot server logging in server.rs represents request data; done means successful POST logs expose parameters sufficient to correlate operations against Nexus while retaining the existing request fields.