CI observability enhancements. (#1193)

- Adds a a `--log-root` option to `swift system start`, propagating the
value as `CONTAINER_LOG_ROOT` to services for logging to files instead
of the OS log facility. This is not a "production" capability as it
neither merges nor rotates logs.
- Currently we don't collect logs on CI builds, and we don't have
permission to run the `log` command there. The PR adds `--log-root` to
the CI test phase, archives the results, and uploads the archive as an
artifact.
- Use FilePath from swift-system for the log root. Foundation URL is a
bit of a footgun for filesystem paths, so unless we identify a
showstopper, we should incrementally transition to this type everywhere
except where we really need network URLs.
- Output the hostname of the CI runner at the start of the test phase so
we can identify runner-specific issues where they exist.
- Fix formatting for log messages with multiple metadata items, and fix
unstructured messages on instances that weren't found using `grep -r
'log\.' Sources`.
- Adds command reference documentation for `--log-root`.

Sources/ContainerXPC/XPCServer.swift

@@ -199,14 +199,24 @@ public struct XPCServer: Sendable {
                 let response = try await handler(message)
                 xpc_connection_send_message(connection, response.underlying)
             } catch let error as ContainerizationError {
-                log.error("route handler threw an error", metadata: ["route": "\(route)", "error": "\(error)"])
+                log.error(
+                    "route handler threw an error",
+                    metadata: [
+                        "route": "\(route)",
+                        "error": "\(error)",
+                    ])
                 Self.replyWithError(
                     connection: connection,
                     object: object,
                     err: error
                 )
             } catch {
-                log.error("route handler threw an error", metadata: ["route": "\(route)", "error": "\(error)"])
+                log.error(
+                    "route handler threw an error",
+                    metadata: [
+                        "route": "\(route)",
+                        "error": "\(error)",
+                    ])
                 let message = XPCMessage(object: object)
                 let reply = message.reply()