Add FAQ about jaeger-agent (#335)

Signed-off-by: Yuri Shkuro <ys@uber.com>

content/docs/1.15/faq.md

@@ -15,3 +15,12 @@ The Dependencies page shows a graph of services traced by Jaeger and connections
 ## Why do I not see any spans in Jaeger?
 
 Please refer to the [Troubleshooting](../troubleshooting/) guide.
+
+## Do I need to run jaeger-agent?
+
+`jaeger-agent` is not always necessary. Jaeger client libraries can be configured to export trace data directly to `jaeger-collector`. However, the following are the reasons why running `jaeger-agent` is recommended:
+
+  * If we want Jaeger client libraries to send trace data directly to collectors, we must provide them with a URL of the HTTP endpoint. It means that our applications require additional configuration containing this parameter, especially if we are running multiple Jaeger installations (e.g. in different availability zones or regions) and want the data sent to a nearby installation. In contrast, when using the agent, the libraries require no additional configuration because the agent is always accessible via `localhost`. It acts as a sidecar and proxies the requests to the appropriate collectors.
+  * The agent can be configured to enrich the tracing data with infrastructure-specific metadata by adding extra tags to the spans, such as the current zone, region, etc. If the agent is running as a host daemon, it will be shared by all applications running on the same host. If the agent is running as a true sidecar, i.e. one per application, it can provide additional functionality such as strong authentication, multi-tenancy (see [this blog post](https://medium.com/jaegertracing/jaeger-and-multitenancy-99dfa1d49dc0)), pod name, etc.
+  * If we want Jaeger client libraries to use sampling strategies that are centrally configured in the collectors, this is only possible by using the `/sampling` HTTP endpoint on the agent. There is no technical reason why this endpoint cannot be implemented directly in the collectors, it's just [not done yet](https://github.com/jaegertracing/jaeger/issues/1971).
+  * Agents allow implementing traffic control to the collectors. If we have thousands of hosts in the data center, each running many applications, and each application sending data directly to the collectors, there may be too many open connections for each collector to handle. The agents can load balance this traffic with fewer connections.

content/docs/next-release/faq.md

@@ -15,3 +15,12 @@ The Dependencies page shows a graph of services traced by Jaeger and connections
 ## Why do I not see any spans in Jaeger?
 
 Please refer to the [Troubleshooting](../troubleshooting/) guide.
+
+## Do I need to run jaeger-agent?
+
+`jaeger-agent` is not always necessary. Jaeger client libraries can be configured to export trace data directly to `jaeger-collector`. However, the following are the reasons why running `jaeger-agent` is recommended:
+
+  * If we want Jaeger client libraries to send trace data directly to collectors, we must provide them with a URL of the HTTP endpoint. It means that our applications require additional configuration containing this parameter, especially if we are running multiple Jaeger installations (e.g. in different availability zones or regions) and want the data sent to a nearby installation. In contrast, when using the agent, the libraries require no additional configuration because the agent is always accessible via `localhost`. It acts as a sidecar and proxies the requests to the appropriate collectors.
+  * The agent can be configured to enrich the tracing data with infrastructure-specific metadata by adding extra tags to the spans, such as the current zone, region, etc. If the agent is running as a host daemon, it will be shared by all applications running on the same host. If the agent is running as a true sidecar, i.e. one per application, it can provide additional functionality such as strong authentication, multi-tenancy (see [this blog post](https://medium.com/jaegertracing/jaeger-and-multitenancy-99dfa1d49dc0)), pod name, etc.
+  * If we want Jaeger client libraries to use sampling strategies that are centrally configured in the collectors, this is only possible by using the `/sampling` HTTP endpoint on the agent. There is no technical reason why this endpoint cannot be implemented directly in the collectors, it's just [not done yet](https://github.com/jaegertracing/jaeger/issues/1971).
+  * Agents allow implementing traffic control to the collectors. If we have thousands of hosts in the data center, each running many applications, and each application sending data directly to the collectors, there may be too many open connections for each collector to handle. The agents can load balance this traffic with fewer connections.