Log errors that occur during /explain (#134)

### What

Make an OpenTelemetry log event when the queries to `/explain` encounter
an error.

### How

OpenTelemetry log events are simply events with the field
`meta.signal_type = "log"`, and otherwise adhering to the semantic
conventions of logs, see
https://opentelemetry.io/docs/specs/otel/logs/data-model/.

Similarly, we set `error = true`, which both Honeycomb and Jaeger
recognizes in the UI.

The `tracing::event!(..)` macro captures the place in the code where it
was called. Therefore I have opted to not try and abstract over making
error-log events, as that information becomes just noise otherwise.

The `body` attribute of the event/log is just the Display'd Error
message. Depending on the future uses we want for these log messages we
will probably amend them with more attributes/ more information.

In Jaeger:

![image](https://github.com/hasura/ndc-postgres/assets/358550/648c9e8a-89bb-4fd1-9bd2-300d7ab095ef)

In Honeycomb:

![image](https://github.com/hasura/ndc-postgres/assets/358550/f3a8b2a3-89d1-497e-b4bb-2a21aa2fc0a3)

crates/connectors/ndc-postgres/src/connector.rs

@@ -134,6 +134,17 @@ impl connector::Connector for Postgres {
         let conf = &configuration.as_runtime_configuration();
         explain::explain(conf, state, query_request)
             .await
+            .map_err(|err| {
+                tracing::error!(
+                    meta.signal_type = "log",
+                    event.domain = "ndc",
+                    event.name = "Explain error",
+                    name = "Explain error",
+                    body = %err,
+                    error = true,
+                );
+                err
+            })
             .map(Into::into)
     }