[wlan] Collect periodic link metrics in ThroughputTest

Before, we sampled link metrics (RSSI, TX PHY rate, RX PHY rate) once
during the test. This CL makes us sample it every second during iperf
throughput measurements, starting 1s after traffic starts and running
until the measurement concludes.

Update the CSV export to report avg, min, and max for each of the AP and
DUT metrics, and print to the log as well.

Introduce some helper classes `MetricStats` and `LinkMetricSummary` to
aggregate sample statistics across each run.

Bug: 556250482
Test: fx build --host //src/connectivity/wlan/tests/core:throughput_test
Test: https://ci.chromium.org/b/8670649944123692529
TAG: agy
CONV: 7ceb8c5e-a9b1-4571-b03e-f215a14530f9
Change-Id: I9ffe43e43a650d8a36cd287a3eab15de6dcd8056
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1825129
Commit-Queue: Marc Khouri <mnck@google.com>
Reviewed-by: Paul Sethi <paulsethi@google.com>
Fuchsia-Auto-Submit: Marc Khouri <mnck@google.com>
Presubmit-Verified: fuchsia-internal-scoped@luci-project-accounts.iam.gserviceaccount.com <fuchsia-internal-scoped@luci-project-accounts.iam.gserviceaccount.com>