[Sui] add a process label to uptime metric (MystenLabs#14951)

## Description 

This helps to differentiate between validators and fullnodes. When
validators have stuck consensus, it is harder to differentiate via other
metrics.

## Test Plan 

CI. Private testnet.

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

crates/mysten-metrics/src/lib.rs

@@ -319,13 +319,17 @@ impl RegistryService {
 }
 
 /// Create a metric that measures the uptime from when this metric was constructed.
-/// The metric is labeled with the provided 'version' label (this should generally be of the
-/// format: 'semver-gitrevision') and the provided 'chain_identifier' label.
+/// The metric is labeled with:
+/// - 'process': the process type, differentiating between validator and fullnode
+/// - 'version': binary version, generally be of the format: 'semver-gitrevision'
+/// - 'chain_identifier': the identifier of the network which this process is part of
 pub fn uptime_metric(
+    process: &str,
     version: &'static str,
     chain_identifier: &str,
 ) -> Box<dyn prometheus::core::Collector> {
     let opts = prometheus::opts!("uptime", "uptime of the node service in seconds")
+        .variable_label("process")
         .variable_label("version")
         .variable_label("chain_identifier");
 
@@ -335,7 +339,7 @@ pub fn uptime_metric(
         opts,
         prometheus_closure_metric::ValueType::Counter,
         uptime,
-        &[version, chain_identifier],
+        &[process, version, chain_identifier],
     )
     .unwrap();
 

crates/sui-node/src/main.rs

@@ -123,12 +123,17 @@ fn main() {
         let node = node_once_cell_clone.get().await;
         let chain_identifier = match node.state().get_chain_identifier() {
             Some(chain_identifier) => chain_identifier.to_string(),
-            None => "Unknown".to_string(),
+            None => "unknown".to_string(),
         };
 
         info!("Sui chain identifier: {chain_identifier}");
         prometheus_registry
             .register(mysten_metrics::uptime_metric(
+                if is_validator {
+                    "validator"
+                } else {
+                    "fullnode"
+                },
                 VERSION,
                 chain_identifier.as_str(),
             ))

crates/sui-proxy/src/main.rs

@@ -91,7 +91,11 @@ async fn main() -> Result<()> {
     let registry_service = metrics::start_prometheus_server(metrics_listener);
     let prometheus_registry = registry_service.default_registry();
     prometheus_registry
-        .register(mysten_metrics::uptime_metric(VERSION, "sui-proxy"))
+        .register(mysten_metrics::uptime_metric(
+            "sui-proxy",
+            VERSION,
+            "unavailable",
+        ))
         .unwrap();
     let app = app(
         Labels {