feat(tracing): Allow trace collector to be unmeshed

The proxy currently treats the trace collector endpoint as a control plane component, mostly to prevent a circular dependency in the service stack. This means that we've had to manually supply a mesh identity in the form of a service account name, usually through an extension's injector.

However, this isn't strictly required as the control plane stack works just fine when the endpoint is unmeshed. This is realistically only useful for the trace collector which don't benefit much from being meshed, so this change only allows the collector to omit a mesh identity.

Signed-off-by: Scott Fleener <scott@buoyant.io>

Author: sfleen

linkerd/app/src/env.rs

@@ -444,7 +444,7 @@ pub fn parse_config<S: Strings>(strings: &S) -> Result<super::Config, EnvError>
     let trace_protocol = strings.get(ENV_TRACE_PROTOCOL);
     let trace_service_name = strings.get(ENV_TRACE_SERVICE_NAME);
 
-    let trace_collector_addr = parse_control_addr(strings, ENV_TRACE_COLLECTOR_SVC_BASE);
+    let trace_collector_addr = parse_optionally_named_control_addr(strings, ENV_TRACE_COLLECTOR_SVC_BASE);
 
     let gateway_suffixes = parse(strings, ENV_INBOUND_GATEWAY_SUFFIXES, parse_dns_suffixes);
 
@@ -1133,6 +1133,21 @@ pub fn parse_backoff<S: Strings>(
 pub fn parse_control_addr<S: Strings>(
     strings: &S,
     base: &str,
+) -> Result<Option<ControlAddr>, EnvError> {
+    parse_control_addr_inner(strings, base, false)
+}
+
+pub fn parse_optionally_named_control_addr<S: Strings>(
+    strings: &S,
+    base: &str,
+) -> Result<Option<ControlAddr>, EnvError> {
+    parse_control_addr_inner(strings, base, true)
+}
+
+fn parse_control_addr_inner<S: Strings>(
+    strings: &S,
+    base: &str,
+    name_optional: bool,
 ) -> Result<Option<ControlAddr>, EnvError> {
     let a = parse(strings, &format!("{base}_ADDR"), parse_addr)?;
     let n = parse(strings, &format!("{base}_NAME"), parse_dns_name)?;
@@ -1142,6 +1157,10 @@ pub fn parse_control_addr<S: Strings>(
             addr: addr.clone(),
             identity: Conditional::None(tls::NoClientTls::Loopback),
         })),
+        (Some(ref addr), None) if name_optional => Ok(Some(ControlAddr {
+            addr: addr.clone(),
+            identity: Conditional::None(tls::NoClientTls::Disabled),
+        })),
         (Some(addr), Some(name)) => Ok(Some(ControlAddr {
             addr,
             identity: Conditional::Some(tls::ClientTls::new(