CP-50078: Instrument xapi-storage-script with tracing (#5808)

* CP-50078: Check if SMAPI observation is enabled

Modifies xapi-storage-script to determine if SMAPI observation is enabled by
checking whether a configuration file exists for it within the file system.

Internally, xapi queries the database to determine if a specific component
should be observed but as xapi-storage-script is an external daemon, we rely
on the file system.

To avoid modifying much of the monadic structure, a configuration record is
used to record whether observation is enabled. It is hoped this less invasive
change will make it simpler to rework xapi-storage-script in the future.

The path of the observer configuration directory is relocated to xapi-consts.

Signed-off-by: Colin James <colin.barr@cloud.com>

* CP-50078: Augment RPC transport to use observer.py

Modifies the RPC transport (fork_exec_rpc) to take a "dbg" parameter that can
encode a traceparent. If observation is enabled and a traceparent can be parsed
from the dbg parameter, the storage plugin's script will be invoked via
observer.py (for automatic instrumentation).

This change does not yet seed each of the transports with the dbg parameter;
the volume, datapath, etc. interfaces - defined in xapi-storage - use the
convention of having dbg as the first parameter for each RPC function.

The act of supplying this dbg parameter to the transport could be done
implicitly, but this would be a contract that is hard to enforce, despite
vastly reducing the pervasiveness of changes to come; each handler will need to
partially apply its received dbg parameter to the RPC transport (e.g.
volume_rpc ~dbg) at each usage site.

The python3 path is relocated from xapi_globs.ml to become a constant within
xapi-consts.

Signed-off-by: Colin James <colin.barr@cloud.com>

* CP-50078: Augment RPC transports for tracing

This change is rather pervasive but it seeds the volume and datapath
RPC transport with the received "dbg" parameter at each usage site.

The change could be done implicitly if we could be promised that "dbg" always
occurs as a first parameter - we would then just modify the transport to be
aware of this contract and not have to partially apply it at each usage site.

Signed-off-by: Colin James <colin.barr@cloud.com>

---------

Signed-off-by: Colin James <colin.barr@cloud.com>

Author: Colin

ocaml/xapi-consts/constants.ml

@@ -399,6 +399,10 @@ let good_ciphersuites =
 
 let verify_certificates_path = "/var/xapi/verify-certificates"
 
+let python3_path = "/usr/bin/python3"
+
+let observer_config_dir = "/etc/xensource/observer"
+
 let observer_component_xapi = "xapi"
 
 let observer_component_xenopsd = "xenopsd"

ocaml/xapi-storage-script/dune

@@ -21,6 +21,7 @@
     sexplib0
     uri
     xapi-backtrace
+    xapi-consts
     xapi-consts.xapi_version
     xapi-idl
     xapi-idl.rrd