app-server: add JSON tracing logs (#12287)

- add `LOG_FORMAT=json` support for app-server tracing logs via
`tracing_subscriber`'s built-in JSON formatter
- keep the default human-readable format unchanged and keep `RUST_LOG`
filtering behavior
- document the env var and update lockfile

MODULE.bazel.lock

@@ -1332,6 +1332,7 @@
       "tracing-error_0.2.1": "{\"dependencies\":[{\"default_features\":false,\"features\":[\"std\"],\"name\":\"tracing\",\"req\":\"^0.1.35\"},{\"default_features\":false,\"features\":[\"registry\",\"fmt\"],\"name\":\"tracing-subscriber\",\"req\":\"^0.3.0\"}],\"features\":{\"default\":[\"traced-error\"],\"traced-error\":[]}}",
       "tracing-log_0.2.0": "{\"dependencies\":[{\"name\":\"ahash\",\"optional\":true,\"req\":\"^0.7.6\"},{\"default_features\":false,\"kind\":\"dev\",\"name\":\"criterion\",\"req\":\"^0.3.6\"},{\"name\":\"log\",\"req\":\"^0.4.17\"},{\"name\":\"lru\",\"optional\":true,\"req\":\"^0.7.7\"},{\"name\":\"once_cell\",\"req\":\"^1.13.0\"},{\"kind\":\"dev\",\"name\":\"tracing\",\"req\":\"^0.1.35\"},{\"name\":\"tracing-core\",\"req\":\"^0.1.28\"}],\"features\":{\"default\":[\"log-tracer\",\"std\"],\"interest-cache\":[\"lru\",\"ahash\"],\"log-tracer\":[],\"std\":[\"log/std\"]}}",
       "tracing-opentelemetry_0.32.1": "{\"dependencies\":[{\"default_features\":false,\"features\":[\"html_reports\"],\"kind\":\"dev\",\"name\":\"criterion\",\"req\":\"^0.5.1\"},{\"name\":\"js-sys\",\"req\":\"^0.3.64\",\"target\":\"cfg(all(target_arch = \\\"wasm32\\\", not(target_os = \\\"wasi\\\")))\"},{\"name\":\"lazy_static\",\"optional\":true,\"req\":\"^1.0.2\"},{\"default_features\":false,\"features\":[\"trace\"],\"name\":\"opentelemetry\",\"req\":\"^0.31.0\"},{\"features\":[\"trace\",\"metrics\"],\"kind\":\"dev\",\"name\":\"opentelemetry\",\"req\":\"^0.31.0\"},{\"features\":[\"metrics\",\"grpc-tonic\"],\"kind\":\"dev\",\"name\":\"opentelemetry-otlp\",\"req\":\"^0.31.0\"},{\"features\":[\"semconv_experimental\"],\"kind\":\"dev\",\"name\":\"opentelemetry-semantic-conventions\",\"req\":\"^0.31.0\"},{\"features\":[\"trace\",\"metrics\"],\"kind\":\"dev\",\"name\":\"opentelemetry-stdout\",\"req\":\"^0.31.0\"},{\"default_features\":false,\"features\":[\"trace\",\"experimental_metrics_custom_reader\",\"testing\"],\"kind\":\"dev\",\"name\":\"opentelemetry_sdk\",\"req\":\"^0.31.0\"},{\"features\":[\"flamegraph\",\"criterion\"],\"kind\":\"dev\",\"name\":\"pprof\",\"req\":\"^0.15.0\",\"target\":\"cfg(not(target_os = \\\"windows\\\"))\"},{\"name\":\"smallvec\",\"optional\":true,\"req\":\"^1.0\"},{\"features\":[\"full\"],\"kind\":\"dev\",\"name\":\"tokio\",\"req\":\"^1\"},{\"default_features\":false,\"features\":[\"std\"],\"name\":\"tracing\",\"req\":\"^0.1.35\"},{\"default_features\":false,\"features\":[\"std\",\"attributes\"],\"kind\":\"dev\",\"name\":\"tracing\",\"req\":\"^0.1.35\"},{\"name\":\"tracing-core\",\"req\":\"^0.1.28\"},{\"kind\":\"dev\",\"name\":\"tracing-error\",\"req\":\"^0.2.0\"},{\"default_features\":false,\"name\":\"tracing-log\",\"optional\":true,\"req\":\"^0.2.0\"},{\"default_features\":false,\"features\":[\"registry\",\"std\"],\"name\":\"tracing-subscriber\",\"req\":\"^0.3.22\"},{\"default_features\":false,\"features\":[\"registry\",\"std\",\"fmt\"],\"kind\":\"dev\",\"name\":\"tracing-subscriber\",\"req\":\"^0.3.0\"},{\"name\":\"web-time\",\"req\":\"^1.0.0\",\"target\":\"cfg(all(target_arch = \\\"wasm32\\\", not(target_os = \\\"wasi\\\")))\"}],\"features\":{\"default\":[\"tracing-log\",\"metrics\"],\"metrics\":[\"opentelemetry/metrics\",\"smallvec\"]}}",
+      "tracing-serde_0.2.0": "{\"dependencies\":[{\"name\":\"serde\",\"req\":\"^1\"},{\"kind\":\"dev\",\"name\":\"serde_json\",\"req\":\"^1\"},{\"name\":\"tracing-core\",\"req\":\"^0.1.28\"},{\"default_features\":false,\"name\":\"valuable-serde\",\"optional\":true,\"req\":\"^0.1.0\",\"target\":\"cfg(tracing_unstable)\"},{\"default_features\":false,\"name\":\"valuable_crate\",\"optional\":true,\"package\":\"valuable\",\"req\":\"^0.1.0\",\"target\":\"cfg(tracing_unstable)\"}],\"features\":{\"valuable\":[\"valuable_crate\",\"valuable-serde\",\"tracing-core/valuable\"]}}",
       "tracing-subscriber_0.3.22": "{\"dependencies\":[{\"default_features\":false,\"features\":[\"clock\",\"std\"],\"name\":\"chrono\",\"optional\":true,\"req\":\"^0.4.26\"},{\"default_features\":false,\"kind\":\"dev\",\"name\":\"criterion\",\"req\":\"^0.3.6\"},{\"kind\":\"dev\",\"name\":\"log\",\"req\":\"^0.4.17\"},{\"name\":\"matchers\",\"optional\":true,\"req\":\"^0.2.0\"},{\"name\":\"nu-ansi-term\",\"optional\":true,\"req\":\"^0.50.0\"},{\"name\":\"once_cell\",\"optional\":true,\"req\":\"^1.13.0\"},{\"name\":\"parking_lot\",\"optional\":true,\"req\":\"^0.12.1\"},{\"default_features\":false,\"features\":[\"std\"],\"kind\":\"dev\",\"name\":\"regex\",\"req\":\"^1\"},{\"default_features\":false,\"features\":[\"std\"],\"name\":\"regex-automata\",\"optional\":true,\"req\":\"^0.4\"},{\"name\":\"serde\",\"optional\":true,\"req\":\"^1.0.140\"},{\"name\":\"serde_json\",\"optional\":true,\"req\":\"^1.0.82\"},{\"name\":\"sharded-slab\",\"optional\":true,\"req\":\"^0.1.4\"},{\"name\":\"smallvec\",\"optional\":true,\"req\":\"^1.9.0\"},{\"name\":\"thread_local\",\"optional\":true,\"req\":\"^1.1.4\"},{\"features\":[\"formatting\"],\"name\":\"time\",\"optional\":true,\"req\":\"^0.3.2\"},{\"features\":[\"formatting\",\"macros\"],\"kind\":\"dev\",\"name\":\"time\",\"req\":\"^0.3.2\"},{\"features\":[\"rt\",\"rt-multi-thread\",\"macros\"],\"kind\":\"dev\",\"name\":\"tokio\",\"req\":\"^1\"},{\"default_features\":false,\"name\":\"tracing\",\"optional\":true,\"req\":\"^0.1.43\"},{\"kind\":\"dev\",\"name\":\"tracing\",\"req\":\"^0.1.43\"},{\"default_features\":false,\"name\":\"tracing-core\",\"req\":\"^0.1.35\"},{\"default_features\":false,\"features\":[\"std-future\",\"std\"],\"kind\":\"dev\",\"name\":\"tracing-futures\",\"req\":\"^0.2.0\"},{\"default_features\":false,\"features\":[\"log-tracer\",\"std\"],\"name\":\"tracing-log\",\"optional\":true,\"req\":\"^0.2.0\"},{\"kind\":\"dev\",\"name\":\"tracing-log\",\"req\":\"^0.2.0\"},{\"name\":\"tracing-serde\",\"optional\":true,\"req\":\"^0.2.0\"},{\"default_features\":false,\"name\":\"valuable-serde\",\"optional\":true,\"req\":\"^0.1.0\",\"target\":\"cfg(tracing_unstable)\"},{\"default_features\":false,\"name\":\"valuable_crate\",\"optional\":true,\"package\":\"valuable\",\"req\":\"^0.1.0\",\"target\":\"cfg(tracing_unstable)\"}],\"features\":{\"alloc\":[],\"ansi\":[\"fmt\",\"nu-ansi-term\"],\"default\":[\"smallvec\",\"fmt\",\"ansi\",\"tracing-log\",\"std\"],\"env-filter\":[\"matchers\",\"once_cell\",\"tracing\",\"std\",\"thread_local\",\"dep:regex-automata\"],\"fmt\":[\"registry\",\"std\"],\"json\":[\"tracing-serde\",\"serde\",\"serde_json\"],\"local-time\":[\"time/local-offset\"],\"nu-ansi-term\":[\"dep:nu-ansi-term\"],\"regex\":[],\"registry\":[\"sharded-slab\",\"thread_local\",\"std\"],\"std\":[\"alloc\",\"tracing-core/std\"],\"valuable\":[\"tracing-core/valuable\",\"valuable_crate\",\"valuable-serde\",\"tracing-serde/valuable\"]}}",
       "tracing-test-macro_0.2.5": "{\"dependencies\":[{\"name\":\"quote\",\"req\":\"^1\"},{\"features\":[\"full\"],\"name\":\"syn\",\"req\":\"^2\"}],\"features\":{\"no-env-filter\":[]}}",
       "tracing-test_0.2.5": "{\"dependencies\":[{\"features\":[\"rt-multi-thread\",\"macros\"],\"kind\":\"dev\",\"name\":\"tokio\",\"req\":\"^1\"},{\"default_features\":false,\"features\":[\"std\"],\"kind\":\"dev\",\"name\":\"tracing\",\"req\":\"^0.1\"},{\"name\":\"tracing-core\",\"req\":\"^0.1\"},{\"features\":[\"env-filter\"],\"name\":\"tracing-subscriber\",\"req\":\"^0.3\"},{\"name\":\"tracing-test-macro\",\"req\":\"^0.2.5\"}],\"features\":{\"no-env-filter\":[\"tracing-test-macro/no-env-filter\"]}}",