Pass tracing context in env variables instead of the spec file. (#4174)

If compute_ctl is launched without a spec file, it fetches it from the
control plane with an HTTP request. We cannot get the startup tracing
context from the compute spec in that case, because we don't have it
available on start. We could still read the tracing context from the
compute spec after we have fetched it, but that would leave the fetch
itself out of the context. Pass the tracing context in environment
variables instead.

libs/compute_api/src/spec.rs

@@ -5,7 +5,6 @@
 //! and connect it to the storage nodes.
 use serde::{Deserialize, Serialize};
 use serde_with::{serde_as, DisplayFromStr};
-use std::collections::HashMap;
 use utils::lsn::Lsn;
 
 /// String type alias representing Postgres identifier and
@@ -31,8 +30,6 @@ pub struct ComputeSpec {
     pub mode: ComputeMode,
 
     pub storage_auth_token: Option<String>,
-
-    pub startup_tracing_context: Option<HashMap<String, String>>,
 }
 
 #[serde_as]