add optional support for tokio-console

This turned out to be quite hairy, mostly because we need to apply the
config's log level filter to the actual logs (stdout and, optionally
sentry), but do not want to filter out the tokio tracing events needed by
the console_subscriber. I hit several edge cases in tracing getting
this to work, and we now depend on a git version of tracing with a
backported patch :(

Cargo.toml

@@ -18,6 +18,8 @@ rust-version = "1.75.0"
 
 
 [dependencies]
+console-subscriber = { version = "0.1", optional = true }
+
 hot-lib-reloader = { version = "^0.6", optional = true }
 
 # Used for secure identifiers
@@ -338,6 +340,18 @@ hardened_malloc-rs = { version = "0.1.2", optional = true, features = [
 #hardened_malloc-rs = { optional = true, features = ["static","clang","light"], path = "../hardened_malloc-rs", default-features = false }
 
 
+# backport of [https://github.com/tokio-rs/tracing/pull/2956] to the 0.1.x branch of tracing.
+# we can switch back to upstream if #2956 is merged and backported in the upstream repo.
+[patch.crates-io.tracing-subscriber]
+git = "https://github.com/Benjamin-L/tracing"
+branch = "tracing-subscriber/env-filter-clone-0.1.x-backport"
+[patch.crates-io.tracing]
+git = "https://github.com/Benjamin-L/tracing"
+branch = "tracing-subscriber/env-filter-clone-0.1.x-backport"
+[patch.crates-io.tracing-core]
+git = "https://github.com/Benjamin-L/tracing"
+branch = "tracing-subscriber/env-filter-clone-0.1.x-backport"
+
 [features]
 default = [
     "backend_rocksdb",
@@ -373,6 +387,10 @@ perf_measurements = [
     "opentelemetry-jaeger",
 ]
 
+# enable the tokio_console server
+# incompatible with release_max_log_level
+tokio_console = ["console-subscriber", "tokio/tracing"]
+
 hot_reload = ["dep:hot-lib-reloader"]
 
 hardened_malloc = ["hardened_malloc-rs"]