context: Replace specific illegal chars

The otel library identifies a certain set of chars
that cannot be added as a value to member, this is to
allow them to be set as http headers. This change check
and replaces those chars with their hex counterparts.

Signed-off-by: crozzy <[email protected]>

Merge pull request #4 from crozzy/crozzy-url-encode-value

context: escape the member value