feat(748): Add compile time feature flag for table metrics

Closes #748.

This patch adds a temporary feature flag for enabling table metrics.

Table metrics are potentially recorded many times within a transaction.
This recording is synchronous and therefore can have a noticable impact on latency.

This new flag will be enabled by default.
With it, table metrics will be recorded; without it, they won't.

Note this flag is temporary.
It will be removed once metrics are recorded async.

Author: joshua-spacetime

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
           sudo chmod 777 /stdb
 
       - name: Run cargo test
-        run: cargo test --all --features odb_rocksdb,odb_sled
+        run: cargo test --all --features odb_rocksdb,odb_sled,metrics
 
   lints:
     name: Lints
@@ -61,7 +61,7 @@ jobs:
         run: cargo fmt --all -- --check
 
       - name: Run cargo clippy
-        run: cargo clippy --all --tests --features odb_rocksdb,odb_sled -- -D warnings
+        run: cargo clippy --all --tests --features odb_rocksdb,odb_sled,metrics -- -D warnings
 
   wasm_bindings:
     name: Build and test wasm bindings

crates/core/Cargo.toml

@@ -93,10 +93,12 @@ wasmtime.workspace = true
 rocksdb = {workspace = true, optional = true}
 
 [features]
+# Optional metrics collected by spacetimedb.
+metrics = []
 # Optional storage engines.
 odb_rocksdb = ["dep:rocksdb"]
 odb_sled = []
-default = ["odb_sled"]
+default = ["odb_sled", "metrics"]
 
 [dev-dependencies]
 spacetimedb-lib = { path = "../lib", features = ["proptest"] }

crates/core/src/db/datastore/locking_tx_datastore/mod.rs

@@ -1978,6 +1978,7 @@ pub struct Iter<'a> {
     num_committed_rows_fetched: u64,
 }
 
+#[cfg(feature = "metrics")]
 impl Drop for Iter<'_> {
     fn drop(&mut self) {
         DB_METRICS
@@ -2111,6 +2112,7 @@ pub struct IndexSeekIterMutTxId<'a> {
     num_committed_rows_fetched: u64,
 }
 
+#[cfg(feature = "metrics")]
 impl Drop for IndexSeekIterMutTxId<'_> {
     fn drop(&mut self) {
         let table_name = self
@@ -2195,6 +2197,7 @@ pub struct CommittedIndexIter<'a> {
     num_committed_rows_fetched: u64,
 }
 
+#[cfg(feature = "metrics")]
 impl Drop for CommittedIndexIter<'_> {
     fn drop(&mut self) {
         let table_name = self