Add admin API docs

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

docs/querying/api.md

@@ -381,3 +381,87 @@ $ curl http://localhost:9090/api/v1/alertmanagers
   }
 }
 ```
+
+
+## TSDB Admin APIs
+In 2.0, there are new APIs that expose database functionalities for the advanced user. These APIs are not enabled unless the `--web.enable-admin-api` is set. All the APIs below are experimental and might change in the future.
+
+We also expose a gRPC API whose definition can be found [here](https://github.com/prometheus/prometheus/blob/master/prompb/rpc.proto).
+
+### Snapshot
+Snapshot creates a snapshot of all current data into `snapshots/<datetime>-<rand>` under the TSDB's data directory and returns the directory as response.
+
+```
+POST /api/v2/admin/tsdb/snapshot
+```
+
+```json
+$ curl -XPOST http://localhost:9090/api/v2/admin/tsdb/snapshot
+{
+  "name": "2017-11-30T15:31:59Z-2366f0a55106d6e1"
+}
+```
+
+The snapshot now exists at `<data-dir>/snapshots/2017-11-30T15:31:59Z-2366f0a55106d6e1`
+
+
+### Delete Series
+DeleteSeries deletes data for a selection of series in a time range. The actual data still exists on disk and is cleaned up in future compactions or can be explicitly cleaned up by hitting the Clean Tombstones endpoint.
+
+```
+POST /api/v2/admin/tsdb/delete_series
+```
+
+Parameters (body):
+
+```json
+{
+    "min_time": "date-time(iso-8601)" // optional, defaults to minmum-time.
+    "max_time": "date-time(iso-8601)" // optional, defaults to maximum-time
+     "matchers": [LabelMatcher]
+}
+```
+
+```json
+LabelMatchers: Matcher specifies a rule, which can match or set of labels or not.
+
+{
+  "type": "string", // One of: EQ, NEQ, RE, NRE defaults to EQ. 
+  "name": "label-name",
+  "value": "label-value"
+}
+```
+
+Example:
+
+```json
+$ curl -X POST \                                                              
+  http://localhost:9090/api/v2/admin/tsdb/delete_series \
+  -H 'content-type: application/json' \
+  -d '{
+        "max_time": "'2017-11-30T20:18:30+05:30",
+        "matchers": [{
+                "type": "EQ",
+                "name": "__name__",
+                "value": "up"
+        }]
+    }'
+    
+{}    
+```
+
+
+
+### Clean Tombstones
+CleanTombstones removes the deleted data from disk and cleans up the existing tombstones. This can be used after deleting series to free up space.
+
+```
+POST /api/v2/admin/tsdb/clean_tombstones
+```
+
+This takes no parameters or body.
+
+```json
+$ curl -XPOST http://localhost:9090/api/v2/admin/tsdb/clean_tombstones
+{}
+```