Add a Cargo optional feature to replace std::time::Instant with tikv/minstant::Instant

[eval-exec]

Is your feature request related to a problem? Please describe.
minstant is drop-in replacement for std::time::Instant that measures time with high performance and high accuracy powered by TSC.

Describe the solution you'd like
Add a cargo feature "minstant" to

rust-prometheus/Cargo.toml

Lines 17 to 22 in f49c724

	[features]
	default = ["protobuf"]
	gen = ["protobuf-codegen-pure"]
	nightly = ["libc"]
	process = ["libc", "procfs"]
	push = ["reqwest", "libc", "protobuf"]

Describe alternatives you've considered

Additional context
In my project, I am using the Histogram module and noticed that it relies on std::time::Instant for time measurement:

rust-prometheus/src/histogram.rs

Lines 517 to 523 in f49c724

	pub fn elapsed(&self) -> Duration {
	match self {
	// We use `saturating_duration_since` to avoid panics caused by non-monotonic clocks.
	Instant::Monotonic(i) => StdInstant::now().saturating_duration_since(*i),
	// It is different from `Instant::Monotonic`, the resolution here is millisecond.

Adding the optional minstant feature to rust-prometheus would be beneficial.

[eval-exec]

Hello, if you think it's a good idea, I would like to open a pull request.