CP-48195: Split tracing library

Attempts to split the tracing library into components and exporter
parts.

While trying to instrument `forkhelpers.ml` with tracing, I found a
cycle dependency: `Tracing(Exporter)` -> `Open_uri` -> `Stunnel` ->
`Forkhelpers`. This splits the tracing library between exporting
functionality and components.

Signed-off-by: Gabriel Buica <danutgabriel.buica@cloud.com>

ocaml/libs/tracing/dune

@@ -1,18 +1,26 @@
 (library
-  (name tracing)
-  (public_name xapi-tracing)
-  (libraries
-    cohttp
-    cohttp-posix
-    ptime
-    ptime.clock.os
-    rpclib.core
-    rpclib.json
-    xapi-log
-    xapi-open-uri
-    xapi-stdext-threads
-    xapi-stdext-unix
-    zstd
-  )
-  (preprocess (pps ppx_deriving_rpc))
-)
+ (name tracing)
+ (modules tracing)
+ (libraries re uri xapi-log xapi-stdext-threads)
+ (public_name xapi-tracing))
+
+(library
+ (name tracing_export)
+ (modules tracing_export)
+ (public_name xapi-tracing.export)
+ (libraries
+  cohttp
+  cohttp-posix
+  ptime
+  ptime.clock.os
+  rpclib.core
+  rpclib.json
+  tracing
+  uri
+  xapi-log
+  xapi-open-uri
+  xapi-stdext-threads
+  xapi-stdext-unix
+  zstd)
+ (preprocess
+  (pps ppx_deriving_rpc)))