Make GPU support optional to enable RISC-V builds without libdrm

- Add 'gpu' feature flag (enabled by default)
- Make gfxinfo dependency optional
- Provide no-op GPU metrics when gpu feature disabled
- Disable GPU support for RISC-V builds in CI (libdrm unavailable)
- All other architectures (amd64, arm64, armhf) still get GPU support

socktop_agent/Cargo.toml

@@ -23,7 +23,7 @@ flate2 = { version = "1", default-features = false, features = ["rust_backend"]
 futures-util = "0.3.31"
 tracing = { version = "0.1", optional = true }
 tracing-subscriber = { version = "0.3", features = ["env-filter"], optional = true }
-gfxinfo = "0.1.2"
+gfxinfo = { version = "0.1.2", optional = true }
 once_cell = "1.19"
 axum-server = { version = "0.7", features = ["tls-rustls"] }
 rustls = { version = "0.23", features = ["aws-lc-rs"] }
@@ -35,7 +35,8 @@ prost = { workspace = true }
 time = { version = "0.3", default-features = false, features = ["formatting", "macros", "parsing" ] }
 
 [features]
-default = []
+default = ["gpu"]
+gpu = ["gfxinfo"]
 logging = ["tracing", "tracing-subscriber"]
 
 [build-dependencies]