[bwplotka]

Adds the optional discovery Cargo feature (serde and serde_json only) with TargetGroup, StaticConfig, DiscoveryConfigs, and Prometheus duration parsing and formatting helpers (parse_duration and format_duration). Part of prometheus/scrape.rs#36.

Stack:

1. discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers #39 discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers (this PR)
2. discovery/dns: add DNS service discovery (dns_sd_configs) #43 discovery/dns: add DNS service discovery (dns_sd_configs)
3. http_config: add Prometheus HTTPClientConfig and reqwest client builder #44 http_config: add Prometheus HTTPClientConfig and reqwest client builder
4. discovery/kubernetes: add pod service discovery (role: pod) #45 discovery/kubernetes: add pod service discovery (role: pod)

Commits in reading order:

1. 4e66d19: discovery: add Prometheus duration parser and formatter
2. fcb3c41: discovery: add TargetGroup, StaticConfig, and DiscoveryConfigs

Intentional differences from Go

• StaticConfig and TargetGroup are distinct Rust structs rather than a slice type alias over targetgroup.Group as in discovery/discovery.go#L150 and discovery/targetgroup/targetgroup.go#L24-L65. Both serialize and deserialize identical JSON wire payloads, including always emitting "targets": [] on empty groups.
• parse_duration and format_duration return unsigned std::time::Duration at millisecond precision instead of signed int64 model.Duration in model/time.go#L227-L294, because discovery refresh intervals are non-negative.