Implement various From and TryFrom for MetadataValue<Ascii>

## Feature Request

### Crates

- tonic

### Motivation

There's many places where the docs say "This function is intended to be replaced in the future by a TryFrom implementation once the trait is stabilized in std" e.g. [MetadataValue::from_str](https://docs.rs/tonic/latest/tonic/metadata/struct.MetadataValue.html#method.from_str). I assume these originally came from copying the Hyper `HeaderValue` methods, which have the same message, e.g. [HeaderValue::from_str](https://docs.rs/hyper/latest/hyper/header/struct.HeaderValue.html#method.from_str).

Integers can be infallibly encoded into ASCII, and so they can be infallibly used as metadata values. But right now there's no API for this -- you have to turn the integer into a string, then use the fallible `MetadataValue::from_str` method.

### Proposal
MetadataValue should impl the same From and TryFrom methods that hyper::HeaderValue does, i.e.

- From: i16, i32, i64, isize, u16, u32, u64, usize
- TryFrom: String, &String, &str

Then we should add a deprecation to the special "try_from" or "from" methods that currently exist, which these new From/TryFrom impls replace.

I'm happy to implement this if y'all like the idea.

### Alternatives

We could leave the current methods in place, but that's a bit less convenient for programmers.