[email] document instrumentation (open-telemetry#388)

* email service doc details

* add require statements

docs/services/emailservice.md

@@ -0,0 +1,76 @@
+# email service
+
+This service will send a confirmation email to the user when an order is placed.
+
+[Email service source](../../src/emailservice/)
+
+## Initialize SDK
+
+You will need to require the core OpenTelemetry SDK and exporter Ruby gems, as
+well as any gem that will be needed for auto-instrumentation libraries
+(ie: Sinatra)
+
+```ruby
+require "opentelemetry/sdk"
+require "opentelemetry/exporter/otlp"
+require "opentelemetry/instrumentation/sinatra"
+```
+
+The Ruby SDK uses OpenTelemetry standard environment variables to configure
+OTLP export, resource attributes, and service name automatically. When
+initializing the OpenTelemetry SDK, you will also specify which
+auto-instrumentation libraries to leverage (ie: Sinatra)
+
+```ruby
+OpenTelemetry::SDK.configure do |c|
+  c.use "OpenTelemetry::Instrumentation::Sinatra"
+end
+```
+
+## Traces
+
+### Add attributes to auto-instrumented spans
+
+Within the execution of auto-instrumented code you can get current span from
+context.
+
+```ruby
+  current_span = OpenTelemetry::Trace.current_span
+```
+
+Adding multiple attributes to a span is accomplished using `add_attributes` on
+the span object.
+
+```ruby
+  current_span.add_attributes({
+    "app.order.id" => data.order.order_id,
+  })
+```
+
+Adding only a single attribute can be accomplished using `set_attribute` on the
+span object.
+
+```ruby
+    span.set_attribute("app.email.recipient", data.email)
+```
+
+### Create new spans
+
+New spans can be created and placed into active context using `in_span` from an
+OpenTelemetry Tracer object. When used in conjunction with a `do..end` block,
+the span will automatically be ended when the block ends execution.
+
+```ruby
+  tracer = OpenTelemetry.tracer_provider.tracer('emailservice')
+  tracer.in_span("send_email") do |span|
+    # logic in context of span here
+  end
+```
+
+## Metrics
+
+TBD
+
+## Logs
+
+TBD