restore metal feature to published crates (risc0#2179)

This feature was removed by risc0#2092. This change restores the metal
features back to published crates to prevent breaking semver.

risc0/cargo-risczero/Cargo.toml

@@ -71,4 +71,5 @@ experimental = [
   "dep:zip",
   "risc0-zkvm/prove",
 ]
+metal = []
 r0vm = ["dep:risc0-r0vm"]

risc0/circuit/recursion-sys/Cargo.toml

@@ -20,3 +20,4 @@ risc0-build-kernel = { workspace = true }
 [features]
 default = []
 cuda = ["dep:sppark", "risc0-sys/cuda"]
+metal = []

risc0/circuit/recursion/Cargo.toml

@@ -61,6 +61,7 @@ cuda = [
   "std",
 ]
 default = ["prove", "test"]
+metal = []
 prove = [
   "dep:cfg-if",
   "dep:downloader",

risc0/circuit/rv32im-sys/Cargo.toml

@@ -20,3 +20,4 @@ risc0-build-kernel = { workspace = true }
 [features]
 default = []
 cuda = ["dep:sppark", "risc0-sys/cuda"]
+metal = []

risc0/circuit/rv32im/Cargo.toml

@@ -52,6 +52,7 @@ cuda = [
   "std",
 ]
 default = ["prove"]
+metal = []
 prove = [
   "dep:bytemuck",
   "dep:cfg-if",

risc0/r0vm/Cargo.toml

@@ -29,3 +29,4 @@ cuda = ["risc0-zkvm/cuda"]
 default = []
 disable-dev-mode = ["risc0-zkvm/disable-dev-mode"]
 dual = ["risc0-zkvm/dual"]
+metal = []

risc0/sys/Cargo.toml

@@ -19,3 +19,4 @@ sppark = { workspace = true, optional = true }
 [features]
 default = []
 cuda = ["dep:cust", "dep:sppark"]
+metal = []

risc0/zkp/Cargo.toml

@@ -57,6 +57,7 @@ features = ["prove", "std"]
 circuit_debug = []
 cuda = ["dep:cust", "prove", "risc0-sys/cuda"]
 default = []
+metal = []
 metal_prefix_products = []
 prove = [
   "dep:ff",

risc0/zkvm/Cargo.toml

@@ -123,6 +123,7 @@ cuda = [
   "risc0-zkp/cuda",
 ]
 dual = []
+metal = []
 default = ["client"]
 disable-dev-mode = []
 # This flag uses the docker environment to build test guests such as multi-test