test-mgs-metrics flake fix: Parse dropshot error response

[jgallagher]

Fixes #7363.

I was able to reproduce this locally by making MGS sleep between starting its Oximeter server and starting to collect metrics from SPs:

+++ b/gateway/src/metrics.rs
@@ -427,6 +427,7 @@ async fn start_pollers(
     .await
     .context("we should never return a fatal error here")?;

+    tokio::time::sleep(std::time::Duration::from_secs(20)).await;
     slog::info!(
         &log,
         "starting to poll SP sensor data every {SP_POLL_INTERVAL:?}"

With that sleep injected and the changes in this PR, the test takes ~30 seconds instead of ~10 seconds, but still passes.