[propagation] Add support for baggage metadata/prop.

[TommyCpp]

W3 baggage spec allows users to pass properties along with baggage. This commit introduces metadata into KeyValueMetadata type. Propagators that supports baggage metadata could use it to propagate data.

For example, user can use insert_with_metadata to add an entry into the baggage.

let mut cc = Baggage::new();
let _ = cc.insert_with_metadata("my-name", "my-value", "test");

Or if the propagation the users uses doesn't support metadata. Users can also use insert

let mut cc = Baggage::new();
let _ = cc.insert_with("my-name", "my-value");

Internally metadata is just an opaque string wrapper with no semantic meanings.
We will assign a metadata for every entry in baggage.

Note that since the Metadata currently has no semantic meanings. We will just take the metadata string from header without any validation and process.

Closes #260

[frigus02]

The logic looks good. Just left a few suggestions about the API.

[jtescher]

Same

[jtescher]

this can probably have metadata_str be a str instead of string, don't need either string conversion or the as str below

[TommyCpp]

Hmm I think in else caluse, the format!(";{}"...) cannot be stored as &str since it will be freed at the end clause if metadata_str doesn't own it.

[jtescher]

oh right you need the prefix. What about something like:

let metadata_str = metadata.as_str().trim();
let metadata_prefix = if metadata_str.is_empty() { "" } else { ";" };
utf8_percent_encode(name.as_str().trim(), FRAGMENT)
    .chain(iter::once("="))
    .chain(utf8_percent_encode(String::from(value).trim(), FRAGMENT))
    .chain(iter::once(metadata_prefix))
    .chain(iter::once(metadata_str))
    .collect()
})

[TommyCpp]

Great idea 👍

[jtescher]

@frigus02 any other thoughts or is this good to merge?

[frigus02]

Looks great. Found one line of commented code. 🙂

[jtescher]

Thanks @TommyCpp