Log as JSON when IPX_LOG_FORMAT=json (#91)

The log was text, so the Grafana dashboard picked lines apart with
regular expressions, and a change of wording would have blanked its
panels. With IPX_LOG_FORMAT=json each line is one JSON object: the
access log carries method, path, route, status and ms as fields (the
route passed from the routing layer in the response's extensions), and
each wire event its ev, feed, new, downloaded, failed, bytes, msg and
the rest (log_wire), beside the old message. The two startup lines that
were println! are logged, so no line breaks the JSON. Text stays the
default, for a terminal. The dashboard reads the fields with Loki's json
parser, and groups requests by route rather than path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -4675,6 +4675,16 @@ dependencies = [
  "web-time",
 ]
 
+[[package]]
+name = "tracing-serde"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "704b1aeb7be0d0a84fc9828cae51dab5970fee5088f83d1dd7ee6f6246fc6ff1"
+dependencies = [
+ "serde",
+ "tracing-core",
+]
+
 [[package]]
 name = "tracing-subscriber"
 version = "0.3.23"
@@ -4685,12 +4695,15 @@ dependencies = [
  "nu-ansi-term",
  "once_cell",
  "regex-automata",
+ "serde",
+ "serde_json",
  "sharded-slab",
  "smallvec",
  "thread_local",
  "tracing",
  "tracing-core",
  "tracing-log",
+ "tracing-serde",
 ]
 
 [[package]]