Enable configuring Prometheus metrics port for local runs

[kate-goldenring]

What this PR does / why we need it:
When running Akri locally (rather than in containers), the Controller and Agent try to use the same address for Prometheus metrics. This enables configuring the port with a METRICS_PORT environment variable creating a scenario like the following:

Run controller (specifying metrics run on localhost:8081):

cargo build -p controller
RUST_LOG=info METRICS_PORT=8081 KUBECONFIG=~/test.cluster.config ./target/debug/controller

Run agent (specifying metrics run on localhost:8082):

cargo build -p agent
sudo DEBUG_ECHO_INSTANCES_SHARED=true ENABLE_DEBUG_ECHO=1 RUST_LOG=info METRICS_PORT=8082 KUBECONFIG=~/test.cluster.config DISCOVERY_HANDLERS_DIRECTORY=~/tmp/akri AGENT_NODE_NAME=myNode HOST_CRICTL_PATH=/usr/bin/crictl HOST_RUNTIME_ENDPOINT=/var/run/dockershim.sock HOST_IMAGE_ENDPOINT=/var/run/dockershim.sock ./target/debug/agent

Special notes for your reviewer:
If approved, I can add notes on configuring this to our development doc.

If applicable:

• this PR has an associated PR with documentation in akri-docs
• this PR contains unit tests
• added code adheres to standard Rust formatting (cargo fmt)
• code builds properly (cargo build)
• code is free of common mistakes (cargo clippy)
• all Akri tests succeed (cargo test)
• inline documentation builds (cargo doc)
• version has been updated appropriately (./version.sh)

[bfjelds]

this seems fine to me. would it make sense to leave run_metrics_server as a no-param method and have this code inside it:

    let port = match std::env::var(METRICS_PORT_LABEL) {
        Ok(p) => p.parse::<u16>()?,
        Err(_) => 8080,
    };

[kate-goldenring]

this seems fine to me. would it make sense to leave run_metrics_server as a no-param method and have this code inside it:

    let port = match std::env::var(METRICS_PORT_LABEL) {
        Ok(p) => p.parse::<u16>()?,
        Err(_) => 8080,
    };

@bfjelds great point. Made the change