Add dev-only query logging support

This PR adds query logging support as an optional feature.
It is only allowed during development/debug builds, and will abort when
used during a `--release` build.

For this feature to be fully activated you also need to se an
environment variable `QUERY_LOGGER=1` to activate the debug log-level
for this crate, else there will be no output.

The reason for this PR is that sometimes it is useful to be able to see
the generated queries, like when debugging an issue, or trying to
optimize a query. Currently i always added this code when needed, but
having this a part of the code could benifit other developers too who
maybe need this.

Cargo.toml

@@ -24,6 +24,11 @@ vendored_openssl = ["openssl/vendored"]
 # Enable MiMalloc memory allocator to replace the default malloc
 # This can improve performance for Alpine builds
 enable_mimalloc = ["mimalloc"]
+# This is a development dependency, and should only be used during development!
+# It enables the usage of the diesel_logger crate, which is able to output the generated queries.
+# You also need to set an env variable `QUERY_LOGGER=1` to fully activate this so you do not have to re-compile
+# if you want to turn off the logging for a specific run.
+query_logger = ["diesel_logger"]
 
 # Enable unstable features, requires nightly
 # Currently only used to enable rusts official ip support
@@ -70,6 +75,7 @@ serde_json = "1.0.87"
 # A safe, extensible ORM and Query builder
 diesel = { version = "2.0.2", features = ["chrono", "r2d2"] }
 diesel_migrations = "2.0.0"
+diesel_logger = { version = "0.2.0", optional = true }
 
 # Bundled SQLite
 libsqlite3-sys = { version = "0.25.2", features = ["bundled"], optional = true }