Create basic othttp.Transport for simple client instrumentation (#678)

* Create basic othttp.Transport for simple client instrumentation

* change props to propagators for clarity

* Add othttp.Transport usage example

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

plugin/othttp/common.go

@@ -0,0 +1,54 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package othttp
+
+import (
+	"net/http"
+
+	"go.opentelemetry.io/otel/api/core"
+	"go.opentelemetry.io/otel/api/trace"
+)
+
+// Attribute keys that can be added to a span.
+const (
+	HostKey       = core.Key("http.host")        // the HTTP host (http.Request.Host)
+	MethodKey     = core.Key("http.method")      // the HTTP method (http.Request.Method)
+	PathKey       = core.Key("http.path")        // the HTTP path (http.Request.URL.Path)
+	URLKey        = core.Key("http.url")         // the HTTP URL (http.Request.URL.String())
+	UserAgentKey  = core.Key("http.user_agent")  // the HTTP user agent (http.Request.UserAgent())
+	RouteKey      = core.Key("http.route")       // the HTTP route (ex: /users/:id)
+	RemoteAddrKey = core.Key("http.remote_addr") // the network address of the client that sent the HTTP request (http.Request.RemoteAddr)
+	StatusCodeKey = core.Key("http.status_code") // if set, the HTTP status
+	ReadBytesKey  = core.Key("http.read_bytes")  // if anything was read from the request body, the total number of bytes read
+	ReadErrorKey  = core.Key("http.read_error")  // If an error occurred while reading a request, the string of the error (io.EOF is not recorded)
+	WroteBytesKey = core.Key("http.wrote_bytes") // if anything was written to the response writer, the total number of bytes written
+	WriteErrorKey = core.Key("http.write_error") // if an error occurred while writing a reply, the string of the error (io.EOF is not recorded)
+)
+
+// Filter is a predicate used to determine whether a given http.request should
+// be traced. A Filter must return true if the request should be traced.
+type Filter func(*http.Request) bool
+
+// Setup basic span attributes before so that they
+// are available to be mutated if needed.
+func setBasicAttributes(span trace.Span, r *http.Request) {
+	span.SetAttributes(
+		HostKey.String(r.Host),
+		MethodKey.String(r.Method),
+		PathKey.String(r.URL.Path),
+		URLKey.String(r.URL.String()),
+		UserAgentKey.String(r.UserAgent()),
+	)
+}