entity-manager: debug logs for loadConfigurations

Enable these logs to keep track of json load and parsing delay.

Tested: on Tyan S8030:

simulate fw update or developer changes:
```
rm /var/configuration/system.json
systemctl restart xyz.openbmc_project.EntityManager
```

journal output for one EM start
```
Jun 13 12:23:46 s8030-bmc-30303035c0c1 systemd[1]: Started Entity Manager.
Jun 13 12:23:54 s8030-bmc-30303035c0c1 entity-manager[19814]: finished loading json configuration in 7668ms
Jun 13 12:24:01 s8030-bmc-30303035c0c1 entity-manager[19814]: finished loading json configuration in 6533ms
...
Jun 13 12:24:16 s8030-bmc-30303035c0c1 entity-manager[19814]: finished loading json configuration in 6838ms
...
```

Repeating the benchmark a few times:

7668ms
6533ms
6838ms

3581ms
3601ms
3443ms

3638ms
3343ms
3381ms

The debug log shows how long the json parsing takes and helps with
future optimizations.

Change-Id: I3a776cabcb09a171f8929ad9052383d20e59a422
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/src/entity_manager/configuration.cpp b/src/entity_manager/configuration.cpp
index 8646f57..bd78f1b 100644
--- a/src/entity_manager/configuration.cpp
+++ b/src/entity_manager/configuration.cpp
@@ -1,6 +1,7 @@
 #include "configuration.hpp"
 
 #include "perform_probe.hpp"
+#include "phosphor-logging/lg2.hpp"
 #include "utils.hpp"
 
 #include <nlohmann/json.hpp>
@@ -35,6 +36,8 @@
 // reads json files out of the filesystem
 bool loadConfigurations(std::list<nlohmann::json>& configurations)
 {
+    const auto start = std::chrono::steady_clock::now();
+
     // find configuration files
     std::vector<std::filesystem::path> jsonPaths;
     if (!findFiles(
@@ -102,6 +105,14 @@
             configurations.emplace_back(data);
         }
     }
+
+    const auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(
+                              std::chrono::steady_clock::now() - start)
+                              .count();
+
+    lg2::debug("Finished loading json configuration in {MILLIS}ms", "MILLIS",
+               duration);
+
     return true;
 }