Add Tags and Fields enums.

This changeset adds the tags and fields enums as they are
currently represented in the java implementation.

Completes #7

opentracing-api/src/field.rs

@@ -0,0 +1,32 @@
+/// The following log fields are recommended for instrumentors who are trying to capture more
+/// information about a logged event. Tracers may expose additional features based on these
+/// standardized data points.
+pub enum Fields {
+    /// The type or "kind" of an error (only for event="error" logs). E.g., "Exception", "OSError".
+    ErrorKind,
+    /// The actual Throwable/Exception/Error object instance itself.
+    ErrorObject,
+    /// A stable identifier for some notable moment in the lifetime of a Span. For instance, a mutex
+    /// lock acquisition or release or the sorts of lifetime events in a browser page load described
+    /// in the Performance.timing specification. E.g., from Zipkin, "cs", "sr", "ss", or "cr". Or,
+    /// more generally, "initialized" or "timed out". For errors, "error".
+    Event,
+    /// A concise, human-readable, one-line message explaining the event. E.g., "Could not connect
+    /// to backend", "Cache invalidation succeeded".
+    Message,
+    /// A stack trace in platform-conventional format; may or may not pertain to an error.
+    Stack,
+}
+
+impl Fields {
+    /// Returns the string representation for the enum reference variant.
+    pub fn as_str(&self) -> &'static str {
+        match *self {
+            Fields::ErrorKind => "error.kind",
+            Fields::ErrorObject => "error.object",
+            Fields::Event => "event",
+            Fields::Message => "message",
+            Fields::Stack => "stack",
+        }
+    }
+}

opentracing-api/src/lib.rs

@@ -1 +1,7 @@
-#![doc(html_root_url = "https://docs.rs/opentracing-api/0.1.0")]
+#![doc(html_root_url = "https://docs.rs/opentracing-api/0.1.0")]
+
+mod tag;
+mod field;
+
+pub use tag::Tags;
+pub use field::Fields;

opentracing-api/src/tag.rs

@@ -0,0 +1,83 @@
+/// The following span tags are recommended for instrumentors who are trying to capture more
+/// semantic information about the spans. Tracers may expose additional features based on these
+/// standardized data points. Tag names follow a general structure of namespacing.
+///
+/// See also [Semantic Conventions](https://github.com/opentracing/specification/blob/master/semantic_conventions.md)
+pub enum Tags {
+    /// A constant for setting the span kind to indicate that it represents a client span.
+    SpanKindClient,
+    /// A constant for setting the span kind to indicate that it represents a server span.
+    SpanKindServer,
+    /// A constant for setting the span kind to indicate that it represents a producer span,
+    /// in a messaging scenario.
+    SpanKindProducer,
+    /// A constant for setting the span kind to indicate that it represents a consumer span,
+    /// in a messaging scenario.
+    SpanKindConsumer,
+    /// HTTP_URL records the url of the incoming request.
+    HttpUrl,
+    /// HTTP_STATUS records the http status code of the response.
+    HttpStatus,
+    /// HTTP_METHOD records the http method. Case-insensitive.
+    HttpMethod,
+    /// PEER_HOST_IPV4 records IPv4 host address of the peer.
+    PeerHostIpv4,
+    /// PEER_HOST_IPV6 records the IPv6 host address of the peer.
+    PeerHostIpv6,
+    /// PEER_SERVICE records the service name of the peer.
+    PeerService,
+    /// PEER_HOSTNAME records the host name of the peer.
+    PeerHostname,
+    /// PEER_PORT records the port number of the peer.
+    PeerPort,
+    /// SAMPLING_PRIORITY determines the priority of sampling this Span.
+    SamplingPriority,
+    /// SPAN_KIND hints at the relationship between spans, e.g. client/server.
+    SpanKind,
+    /// COMPONENT is a low-cardinality identifier of the module, library, or
+    /// package that is instrumented.
+    Component,
+    /// ERROR indicates whether a Span ended in an error state.
+    Error,
+    /// DB_TYPE indicates the type of Database.
+    DbType,
+    /// DB_INSTANCE indicates the instance name of Database.
+    DbInstance,
+    /// DB_USER indicates the user name of Database, e.g. "readonly_user" or
+    /// "reporting_user"
+    DbUser,
+    /// DB_STATEMENT records a database statement for the given database type.
+    DbStatement,
+    /// MESSAGE_BUS_DESTINATION records an address at which messages can be
+    /// exchanged.
+    MessageBusDestination,
+}
+
+impl Tags {
+    /// Returns the string representation for the enum reference variant.
+    pub fn as_str(&self) -> &'static str {
+        match *self {
+            Tags::SpanKindClient => "client",
+            Tags::SpanKindServer => "server",
+            Tags::SpanKindProducer => "producer",
+            Tags::SpanKindConsumer => "consumer",
+            Tags::HttpUrl => "http.url",
+            Tags::HttpStatus => "http.status_code",
+            Tags::HttpMethod => "http.method",
+            Tags::PeerHostIpv4 => "peer.ipv4",
+            Tags::PeerHostIpv6 => "peer.ipv6",
+            Tags::PeerService => "peer.service",
+            Tags::PeerHostname => "peer.hostname",
+            Tags::PeerPort => "peer.port",
+            Tags::SamplingPriority => "sampling.priority",
+            Tags::SpanKind => "span.kind",
+            Tags::Component => "component",
+            Tags::Error => "error",
+            Tags::DbType => "db.type",
+            Tags::DbInstance => "db.instance",
+            Tags::DbUser => "db.user",
+            Tags::DbStatement => "db.statement",
+            Tags::MessageBusDestination => "message_bus.destination",
+        }
+    }
+}