Add filters for othttp plugin (#556)

* Add request filtering capability to othhtp.Handler

* Add simple and useful filters for othttp plugin

* Add note that all requests are traced in the absence of any filters

* Add copyright notice to plugin/othttp/filters/filters_test.go

Co-Authored-By: Tyler Yahn <MrAlias@users.noreply.github.com>

* Add package docstring for filters package

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Rahul Patel <rahulpa@google.com>

plugin/othttp/filters/filters.go

@@ -0,0 +1,154 @@
+// Copyright 2020, 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 filters provides a set of filters useful with the
+// othttp.WithFilter() option to control which inbound requests are traced.
+package filters
+
+import (
+	"net/http"
+	"strings"
+
+	"go.opentelemetry.io/otel/plugin/othttp"
+)
+
+// Any takes a list of Filters and returns a Filter that
+// returns true if any Filter in the list returns true.
+func Any(fs ...othttp.Filter) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, f := range fs {
+			if f(r) {
+				return true
+			}
+		}
+		return false
+	}
+}
+
+// All takes a list of Filters and returns a Filter that
+// returns true only if all Filters in the list return true.
+func All(fs ...othttp.Filter) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, f := range fs {
+			if !f(r) {
+				return false
+			}
+		}
+		return true
+	}
+}
+
+// None takes a list of Filters and returns a Filter that returns
+// true only if none of the Filters in the list return true.
+func None(fs ...othttp.Filter) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, f := range fs {
+			if f(r) {
+				return false
+			}
+		}
+		return true
+	}
+}
+
+// Not provides a convenience mechanism for inverting a Filter
+func Not(f othttp.Filter) othttp.Filter {
+	return func(r *http.Request) bool {
+		return !f(r)
+	}
+}
+
+// Hostname returns a Filter that returns true if the request's
+// hostname matches the provided string.
+func Hostname(h string) othttp.Filter {
+	return func(r *http.Request) bool {
+		return r.URL.Hostname() == h
+	}
+}
+
+// Path returns a Filter that returns true if the request's
+// path matches the provided string.
+func Path(p string) othttp.Filter {
+	return func(r *http.Request) bool {
+		return r.URL.Path == p
+	}
+}
+
+// PathPrefix returns a Filter that returns true if the request's
+// path starts with the provided string.
+func PathPrefix(p string) othttp.Filter {
+	return func(r *http.Request) bool {
+		return strings.HasPrefix(r.URL.Path, p)
+	}
+}
+
+// Query returns a Filter that returns true if the request
+// includes a query parameter k with a value equal to v.
+func Query(k, v string) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, qv := range r.URL.Query()[k] {
+			if v == qv {
+				return true
+			}
+		}
+		return false
+	}
+}
+
+// QueryContains returns a Filter that returns true if the request
+// includes a query parameter k with a value that contains v.
+func QueryContains(k, v string) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, qv := range r.URL.Query()[k] {
+			if strings.Contains(qv, v) {
+				return true
+			}
+		}
+		return false
+	}
+}
+
+// Method returns a Filter that returns true if the request
+// method is equal to the provided value.
+func Method(m string) othttp.Filter {
+	return func(r *http.Request) bool {
+		return m == r.Method
+	}
+}
+
+// Header returns a Filter that returns true if the request
+// includes a header k with a value equal to v.
+func Header(k, v string) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, hv := range r.Header.Values(k) {
+			if v == hv {
+				return true
+			}
+		}
+		return false
+	}
+}
+
+// HeaderContains returns a Filter that returns true if the request
+// includes a header k with a value that contains v.
+func HeaderContains(k, v string) othttp.Filter {
+	return func(r *http.Request) bool {
+		for _, hv := range r.Header.Values(k) {
+			if strings.Contains(hv, v) {
+				return true
+			}
+		}
+		return false
+	}
+}