improve enum logging

Ensure we're using an enumeration string everywhere we log one to
improve readability of the log statements.

This depends on I6850b0bb142f0ef5219a5fc07c2cb4e2c90d5779 from
phosphor-logging for `to_string` conversion support.

Tested:

Running directly on development system shows:
```
<7> TYPE=CPU, NAME=CPU SUBTYPE=CPU PATH=, FREQ=1, WSIZE=120
<7> THRESHOLD TYPE=xyz.openbmc_project.Common.Threshold.Type.Warning THRESHOLD BOUND=xyz.openbmc_project.Common.Threshold.Bound.Upper VALUE=80.000000 LOG=False TARGET=
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5ea7a050e94833af272e79e6add716d1d3e66571
diff --git a/health_metric_config.hpp b/health_metric_config.hpp
index 49f39ad..6391462 100644
--- a/health_metric_config.hpp
+++ b/health_metric_config.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <sdbusplus/message.hpp>
 #include <xyz/openbmc_project/Common/Threshold/server.hpp>
 
 #include <chrono>
@@ -41,6 +42,9 @@
     NA
 };
 
+auto to_string(Type) -> std::string;
+auto to_string(SubType) -> std::string;
+
 namespace config
 {