Add actions to list nodes and retrieve node metrics

[CGodiksen]

This PR adds support for two new actions, ListNodes and NodeMetrics. ListNodes lists all the nodes in the cluster, or the node itself if it is a single node deployment. NodeMetrics returns a collection of metrics that show general system metrics such as CPU, disk, and memory usage, and ModelarDB specific metrics such as storage engine memory usage.

Methods have also been added to Client in modelardb_embedded to support node related operations on the client. Note that the client still implements the Operations trait for functionality that is shared between DataFolder and Client so these new methods are only extra functionality that only a Client supports such as FlushNode. Someone using the Rust API for Client now has access to all Apache Arrow Flight server functionality through a single API, instead of having to use Client for some things and a manual Apache Arrow Flight client for other things.

After the new Client was implemented, it was considered if it could be used elsewhere in the system since it now supports all the Apache Arrow Flight functionality. An issue (#417) has been created to explore this further.

[Copilot]

Pull request overview

This pull request extends the ModelarDB Arrow Flight protocol and server/client implementations to support node-oriented operations, specifically listing cluster nodes and retrieving per-node resource/storage-engine metrics.

Changes:

• Added protobuf messages and Rust helpers for encoding/decoding cluster node metadata.
• Implemented new Arrow Flight actions ListNodes and NodeMetrics on the server (and added integration tests).
• Extended modelardb_embedded::Client with node-related APIs (configuration, flush/kill, list nodes, node metrics), plus supporting error handling.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
crates/modelardb_types/src/flight/protocol.proto	Adds NodeMetadata, ClusterNodes, and NodeMetrics protobuf messages.
crates/modelardb_types/src/flight/mod.rs	Adds cluster-node encode/decode helpers and a unit test.
crates/modelardb_server/tests/integration_test.rs	Adds integration tests for ListNodes/NodeMetrics and updates action-list expectations.
crates/modelardb_server/src/storage/types.rs	Makes remaining_ingested_memory_in_bytes available outside tests.
crates/modelardb_server/src/storage/mod.rs	Exposes remaining-memory getters on StorageEngine for metrics reporting.
crates/modelardb_server/src/remote/mod.rs	Implements ListNodes and NodeMetrics actions and updates ClusterMode usage.
crates/modelardb_server/src/main.rs	Removes local ClusterMode definition and uses cluster::ClusterMode.
crates/modelardb_server/src/data_folders.rs	Stores the single-node’s Node metadata inside ClusterMode::SingleNode.
crates/modelardb_server/src/context.rs	Implements metric collection via sysinfo and disk-space identification logic.
crates/modelardb_server/src/configuration.rs	Updates imports/construction to use the new cluster::ClusterMode.
crates/modelardb_server/src/cluster.rs	Defines ClusterMode and adds nodes() helpers for single-node vs cluster.
crates/modelardb_server/Cargo.toml	Adds sysinfo dependency for metrics collection.
crates/modelardb_embedded/src/operations/client.rs	Adds node-related Client methods and refactors action sending/receiving.
crates/modelardb_embedded/src/error.rs	Adds a dedicated Prost decode error variant.
crates/modelardb_embedded/Cargo.toml	Adds prost dependency needed for decoding response protobufs.
Cargo.lock	Updates lockfile for new dependencies (sysinfo, prost).

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